chore(io): Migrate pkg/build to Medium abstraction

- Updated io.Medium interface with Open() and Create() methods to support streaming.
- Migrated pkg/build, pkg/build/builders, and pkg/build/signing to use io.Medium.
- Added FS field to build.Config and updated build.Builder interface.
- Refactored checksum and archive logic to use io.Medium streaming.
- Updated pkg/release and pkg/build/buildcmd to use io.Local.
- Updated unit tests to match new signatures.

pkg/build/archive.go

@@ -8,11 +8,11 @@ import (
 	"compress/gzip"
 	"fmt"
 	"io"
-	"os"
 	"path/filepath"
 	"strings"
 
 	"github.com/Snider/Borg/pkg/compress"
+	io_interface "github.com/host-uk/core/pkg/io"
 )
 
 // ArchiveFormat specifies the compression format for archives.
@@ -31,28 +31,28 @@ const (
 // Uses tar.gz for linux/darwin and zip for windows.
 // The archive is created alongside the binary (e.g., dist/myapp_linux_amd64.tar.gz).
 // Returns a new Artifact with Path pointing to the archive.
-func Archive(artifact Artifact) (Artifact, error) {
-	return ArchiveWithFormat(artifact, ArchiveFormatGzip)
+func Archive(fs io_interface.Medium, artifact Artifact) (Artifact, error) {
+	return ArchiveWithFormat(fs, artifact, ArchiveFormatGzip)
 }
 
 // ArchiveXZ creates an archive for a single artifact using xz compression.
 // Uses tar.xz for linux/darwin and zip for windows.
 // Returns a new Artifact with Path pointing to the archive.
-func ArchiveXZ(artifact Artifact) (Artifact, error) {
-	return ArchiveWithFormat(artifact, ArchiveFormatXZ)
+func ArchiveXZ(fs io_interface.Medium, artifact Artifact) (Artifact, error) {
+	return ArchiveWithFormat(fs, artifact, ArchiveFormatXZ)
 }
 
 // ArchiveWithFormat creates an archive for a single artifact with the specified format.
 // Uses tar.gz or tar.xz for linux/darwin and zip for windows.
 // The archive is created alongside the binary (e.g., dist/myapp_linux_amd64.tar.xz).
 // Returns a new Artifact with Path pointing to the archive.
-func ArchiveWithFormat(artifact Artifact, format ArchiveFormat) (Artifact, error) {
+func ArchiveWithFormat(fs io_interface.Medium, artifact Artifact, format ArchiveFormat) (Artifact, error) {
 	if artifact.Path == "" {
 		return Artifact{}, fmt.Errorf("build.Archive: artifact path is empty")
 	}
 
 	// Verify the source file exists
-	info, err := os.Stat(artifact.Path)
+	info, err := fs.Stat(artifact.Path)
 	if err != nil {
 		return Artifact{}, fmt.Errorf("build.Archive: source file not found: %w", err)
 	}
@@ -62,7 +62,7 @@ func ArchiveWithFormat(artifact Artifact, format ArchiveFormat) (Artifact, error
 
 	// Determine archive type based on OS and format
 	var archivePath string
-	var archiveFunc func(src, dst string) error
+	var archiveFunc func(fs io_interface.Medium, src, dst string) error
 
 	if artifact.OS == "windows" {
 		archivePath = archiveFilename(artifact, ".zip")
@@ -79,7 +79,7 @@ func ArchiveWithFormat(artifact Artifact, format ArchiveFormat) (Artifact, error
 	}
 
 	// Create the archive
-	if err := archiveFunc(artifact.Path, archivePath); err != nil {
+	if err := archiveFunc(fs, artifact.Path, archivePath); err != nil {
 		return Artifact{}, fmt.Errorf("build.Archive: failed to create archive: %w", err)
 	}
 
@@ -93,26 +93,26 @@ func ArchiveWithFormat(artifact Artifact, format ArchiveFormat) (Artifact, error
 
 // ArchiveAll archives all artifacts using gzip compression.
 // Returns a slice of new artifacts pointing to the archives.
-func ArchiveAll(artifacts []Artifact) ([]Artifact, error) {
-	return ArchiveAllWithFormat(artifacts, ArchiveFormatGzip)
+func ArchiveAll(fs io_interface.Medium, artifacts []Artifact) ([]Artifact, error) {
+	return ArchiveAllWithFormat(fs, artifacts, ArchiveFormatGzip)
 }
 
 // ArchiveAllXZ archives all artifacts using xz compression.
 // Returns a slice of new artifacts pointing to the archives.
-func ArchiveAllXZ(artifacts []Artifact) ([]Artifact, error) {
-	return ArchiveAllWithFormat(artifacts, ArchiveFormatXZ)
+func ArchiveAllXZ(fs io_interface.Medium, artifacts []Artifact) ([]Artifact, error) {
+	return ArchiveAllWithFormat(fs, artifacts, ArchiveFormatXZ)
 }
 
 // ArchiveAllWithFormat archives all artifacts with the specified format.
 // Returns a slice of new artifacts pointing to the archives.
-func ArchiveAllWithFormat(artifacts []Artifact, format ArchiveFormat) ([]Artifact, error) {
+func ArchiveAllWithFormat(fs io_interface.Medium, artifacts []Artifact, format ArchiveFormat) ([]Artifact, error) {
 	if len(artifacts) == 0 {
 		return nil, nil
 	}
 
 	var archived []Artifact
 	for _, artifact := range artifacts {
-		arch, err := ArchiveWithFormat(artifact, format)
+		arch, err := ArchiveWithFormat(fs, artifact, format)
 		if err != nil {
 			return archived, fmt.Errorf("build.ArchiveAll: failed to archive %s: %w", artifact.Path, err)
 		}
@@ -142,9 +142,9 @@ func archiveFilename(artifact Artifact, ext string) string {
 
 // createTarXzArchive creates a tar.xz archive containing a single file.
 // Uses Borg's compress package for xz compression.
-func createTarXzArchive(src, dst string) error {
+func createTarXzArchive(fs io_interface.Medium, src, dst string) error {
 	// Open the source file
-	srcFile, err := os.Open(src)
+	srcFile, err := fs.Open(src)
 	if err != nil {
 		return fmt.Errorf("failed to open source file: %w", err)
 	}
@@ -185,7 +185,7 @@ func createTarXzArchive(src, dst string) error {
 	}
 
 	// Write to destination file
-	if err := os.WriteFile(dst, xzData, 0644); err != nil {
+	if err := fs.Write(dst, string(xzData)); err != nil {
 		return fmt.Errorf("failed to write archive file: %w", err)
 	}
 
@@ -193,9 +193,9 @@ func createTarXzArchive(src, dst string) error {
 }
 
 // createTarGzArchive creates a tar.gz archive containing a single file.
-func createTarGzArchive(src, dst string) error {
+func createTarGzArchive(fs io_interface.Medium, src, dst string) error {
 	// Open the source file
-	srcFile, err := os.Open(src)
+	srcFile, err := fs.Open(src)
 	if err != nil {
 		return fmt.Errorf("failed to open source file: %w", err)
 	}
@@ -207,7 +207,7 @@ func createTarGzArchive(src, dst string) error {
 	}
 
 	// Create the destination file
-	dstFile, err := os.Create(dst)
+	dstFile, err := fs.Create(dst)
 	if err != nil {
 		return fmt.Errorf("failed to create archive file: %w", err)
 	}
@@ -243,9 +243,9 @@ func createTarGzArchive(src, dst string) error {
 }
 
 // createZipArchive creates a zip archive containing a single file.
-func createZipArchive(src, dst string) error {
+func createZipArchive(fs io_interface.Medium, src, dst string) error {
 	// Open the source file
-	srcFile, err := os.Open(src)
+	srcFile, err := fs.Open(src)
 	if err != nil {
 		return fmt.Errorf("failed to open source file: %w", err)
 	}
@@ -257,7 +257,7 @@ func createZipArchive(src, dst string) error {
 	}
 
 	// Create the destination file
-	dstFile, err := os.Create(dst)
+	dstFile, err := fs.Create(dst)
 	if err != nil {
 		return fmt.Errorf("failed to create archive file: %w", err)
 	}

pkg/build/archive_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/Snider/Borg/pkg/compress"
+	io_interface "github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -37,6 +38,7 @@ func setupArchiveTestFile(t *testing.T, name, os_, arch string) (binaryPath stri
 }
 
 func TestArchive_Good(t *testing.T) {
+	fs := io_interface.Local
 	t.Run("creates tar.gz for linux", func(t *testing.T) {
 		binaryPath, outputDir := setupArchiveTestFile(t, "myapp", "linux", "amd64")
 
@@ -46,7 +48,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		require.NoError(t, err)
 
 		// Verify archive was created
@@ -71,7 +73,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "arm64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		require.NoError(t, err)
 
 		expectedPath := filepath.Join(outputDir, "myapp_darwin_arm64.tar.gz")
@@ -90,7 +92,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		require.NoError(t, err)
 
 		// Windows archives should strip .exe from archive name
@@ -111,7 +113,7 @@ func TestArchive_Good(t *testing.T) {
 			Checksum: "abc123",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		require.NoError(t, err)
 		assert.Equal(t, "abc123", result.Checksum)
 	})
@@ -125,7 +127,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := ArchiveXZ(artifact)
+		result, err := ArchiveXZ(fs, artifact)
 		require.NoError(t, err)
 
 		expectedPath := filepath.Join(outputDir, "myapp_linux_amd64.tar.xz")
@@ -144,7 +146,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "arm64",
 		}
 
-		result, err := ArchiveWithFormat(artifact, ArchiveFormatXZ)
+		result, err := ArchiveWithFormat(fs, artifact, ArchiveFormatXZ)
 		require.NoError(t, err)
 
 		expectedPath := filepath.Join(outputDir, "myapp_darwin_arm64.tar.xz")
@@ -163,7 +165,7 @@ func TestArchive_Good(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := ArchiveWithFormat(artifact, ArchiveFormatXZ)
+		result, err := ArchiveWithFormat(fs, artifact, ArchiveFormatXZ)
 		require.NoError(t, err)
 
 		// Windows should still get .zip regardless of format
@@ -176,6 +178,7 @@ func TestArchive_Good(t *testing.T) {
 }
 
 func TestArchive_Bad(t *testing.T) {
+	fs := io_interface.Local
 	t.Run("returns error for empty path", func(t *testing.T) {
 		artifact := Artifact{
 			Path: "",
@@ -183,7 +186,7 @@ func TestArchive_Bad(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "artifact path is empty")
 		assert.Empty(t, result.Path)
@@ -196,7 +199,7 @@ func TestArchive_Bad(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "source file not found")
 		assert.Empty(t, result.Path)
@@ -211,7 +214,7 @@ func TestArchive_Bad(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Archive(artifact)
+		result, err := Archive(fs, artifact)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "source path is a directory")
 		assert.Empty(t, result.Path)
@@ -219,6 +222,7 @@ func TestArchive_Bad(t *testing.T) {
 }
 
 func TestArchiveAll_Good(t *testing.T) {
+	fs := io_interface.Local
 	t.Run("archives multiple artifacts", func(t *testing.T) {
 		outputDir := t.TempDir()
 
@@ -255,7 +259,7 @@ func TestArchiveAll_Good(t *testing.T) {
 			})
 		}
 
-		results, err := ArchiveAll(artifacts)
+		results, err := ArchiveAll(fs, artifacts)
 		require.NoError(t, err)
 		require.Len(t, results, 4)
 
@@ -268,19 +272,20 @@ func TestArchiveAll_Good(t *testing.T) {
 	})
 
 	t.Run("returns nil for empty slice", func(t *testing.T) {
-		results, err := ArchiveAll([]Artifact{})
+		results, err := ArchiveAll(fs, []Artifact{})
 		assert.NoError(t, err)
 		assert.Nil(t, results)
 	})
 
 	t.Run("returns nil for nil slice", func(t *testing.T) {
-		results, err := ArchiveAll(nil)
+		results, err := ArchiveAll(fs, nil)
 		assert.NoError(t, err)
 		assert.Nil(t, results)
 	})
 }
 
 func TestArchiveAll_Bad(t *testing.T) {
+	fs := io_interface.Local
 	t.Run("returns partial results on error", func(t *testing.T) {
 		binaryPath, _ := setupArchiveTestFile(t, "myapp", "linux", "amd64")
 
@@ -289,7 +294,7 @@ func TestArchiveAll_Bad(t *testing.T) {
 			{Path: "/nonexistent/binary", OS: "linux", Arch: "arm64"}, // This will fail
 		}
 
-		results, err := ArchiveAll(artifacts)
+		results, err := ArchiveAll(fs, artifacts)
 		assert.Error(t, err)
 		// Should have the first successful result
 		assert.Len(t, results, 1)

pkg/build/build.go

@@ -5,6 +5,8 @@ package build
 
 import (
 	"context"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // ProjectType represents a detected project type.
@@ -49,6 +51,8 @@ type Artifact struct {
 
 // Config holds build configuration.
 type Config struct {
+	// FS is the medium used for file operations.
+	FS io.Medium
 	// ProjectDir is the root directory of the project.
 	ProjectDir string
 	// OutputDir is where build artifacts are placed.
@@ -78,7 +82,7 @@ type Builder interface {
 	// Name returns the builder's identifier.
 	Name() string
 	// Detect checks if this builder can handle the project in the given directory.
-	Detect(dir string) (bool, error)
+	Detect(fs io.Medium, dir string) (bool, error)
 	// Build compiles the project for the specified targets.
 	Build(ctx context.Context, cfg *Config, targets []Target) ([]Artifact, error)
 }

pkg/build/buildcmd/cmd_project.go

@@ -18,10 +18,14 @@ import (
 	"github.com/host-uk/core/pkg/build/builders"
 	"github.com/host-uk/core/pkg/build/signing"
 	"github.com/host-uk/core/pkg/i18n"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // runProjectBuild handles the main `core build` command with auto-detection.
 func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targetsFlag string, outputDir string, doArchive bool, doChecksum bool, configPath string, format string, push bool, imageName string, noSign bool, notarize bool, verbose bool) error {
+	// Use local filesystem as the default medium
+	fs := io.Local
+
 	// Get current working directory as project root
 	projectDir, err := os.Getwd()
 	if err != nil {
@@ -29,7 +33,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 	}
 
 	// Load configuration from .core/build.yaml (or defaults)
-	buildCfg, err := build.LoadConfig(projectDir)
+	buildCfg, err := build.LoadConfig(fs, projectDir)
 	if err != nil {
 		return fmt.Errorf("%s: %w", i18n.T("common.error.failed", map[string]any{"Action": "load config"}), err)
 	}
@@ -39,7 +43,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 	if buildType != "" {
 		projectType = build.ProjectType(buildType)
 	} else {
-		projectType, err = build.PrimaryType(projectDir)
+		projectType, err = build.PrimaryType(fs, projectDir)
 		if err != nil {
 			return fmt.Errorf("%s: %w", i18n.T("common.error.failed", map[string]any{"Action": "detect project type"}), err)
 		}
@@ -98,6 +102,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 
 	// Create build config for the builder
 	cfg := &build.Config{
+		FS:         fs,
 		ProjectDir: projectDir,
 		OutputDir:  outputDir,
 		Name:       binaryName,
@@ -161,7 +166,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 			signingArtifacts[i] = signing.Artifact{Path: a.Path, OS: a.OS, Arch: a.Arch}
 		}
 
-		if err := signing.SignBinaries(ctx, signCfg, signingArtifacts); err != nil {
+		if err := signing.SignBinaries(ctx, fs, signCfg, signingArtifacts); err != nil {
 			if !ciMode {
 				fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("cmd.build.error.signing_failed"), err)
 			}
@@ -169,7 +174,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 		}
 
 		if signCfg.MacOS.Notarize {
-			if err := signing.NotarizeBinaries(ctx, signCfg, signingArtifacts); err != nil {
+			if err := signing.NotarizeBinaries(ctx, fs, signCfg, signingArtifacts); err != nil {
 				if !ciMode {
 					fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("cmd.build.error.notarization_failed"), err)
 				}
@@ -186,7 +191,7 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 			fmt.Printf("%s %s\n", buildHeaderStyle.Render(i18n.T("cmd.build.label.archive")), i18n.T("cmd.build.creating_archives"))
 		}
 
-		archivedArtifacts, err = build.ArchiveAll(artifacts)
+		archivedArtifacts, err = build.ArchiveAll(fs, artifacts)
 		if err != nil {
 			if !ciMode {
 				fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("cmd.build.error.archive_failed"), err)
@@ -256,12 +261,13 @@ func runProjectBuild(ctx context.Context, buildType string, ciMode bool, targets
 
 // computeAndWriteChecksums computes checksums for artifacts and writes CHECKSUMS.txt.
 func computeAndWriteChecksums(ctx context.Context, projectDir, outputDir string, artifacts []build.Artifact, signCfg signing.SignConfig, ciMode bool, verbose bool) ([]build.Artifact, error) {
+	fs := io.Local
 	if verbose && !ciMode {
 		fmt.Println()
 		fmt.Printf("%s %s\n", buildHeaderStyle.Render(i18n.T("cmd.build.label.checksum")), i18n.T("cmd.build.computing_checksums"))
 	}
 
-	checksummedArtifacts, err := build.ChecksumAll(artifacts)
+	checksummedArtifacts, err := build.ChecksumAll(fs, artifacts)
 	if err != nil {
 		if !ciMode {
 			fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("cmd.build.error.checksum_failed"), err)
@@ -271,7 +277,7 @@ func computeAndWriteChecksums(ctx context.Context, projectDir, outputDir string,
 
 	// Write CHECKSUMS.txt
 	checksumPath := filepath.Join(outputDir, "CHECKSUMS.txt")
-	if err := build.WriteChecksumFile(checksummedArtifacts, checksumPath); err != nil {
+	if err := build.WriteChecksumFile(fs, checksummedArtifacts, checksumPath); err != nil {
 		if !ciMode {
 			fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("common.error.failed", map[string]any{"Action": "write CHECKSUMS.txt"}), err)
 		}
@@ -280,7 +286,7 @@ func computeAndWriteChecksums(ctx context.Context, projectDir, outputDir string,
 
 	// Sign checksums with GPG
 	if signCfg.Enabled {
-		if err := signing.SignChecksums(ctx, signCfg, checksumPath); err != nil {
+		if err := signing.SignChecksums(ctx, fs, signCfg, checksumPath); err != nil {
 			if !ciMode {
 				fmt.Printf("%s %s: %v\n", buildErrorStyle.Render(i18n.T("common.label.error")), i18n.T("cmd.build.error.gpg_signing_failed"), err)
 			}

pkg/build/builders/docker.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // DockerBuilder builds Docker images.
@@ -26,9 +27,9 @@ func (b *DockerBuilder) Name() string {
 }
 
 // Detect checks if a Dockerfile exists in the directory.
-func (b *DockerBuilder) Detect(dir string) (bool, error) {
+func (b *DockerBuilder) Detect(fs io.Medium, dir string) (bool, error) {
 	dockerfilePath := filepath.Join(dir, "Dockerfile")
-	if _, err := os.Stat(dockerfilePath); err == nil {
+	if fs.IsFile(dockerfilePath) {
 		return true, nil
 	}
 	return false, nil
@@ -53,7 +54,7 @@ func (b *DockerBuilder) Build(ctx context.Context, cfg *build.Config, targets []
 	}
 
 	// Validate Dockerfile exists
-	if _, err := os.Stat(dockerfile); err != nil {
+	if !cfg.FS.IsFile(dockerfile) {
 		return nil, fmt.Errorf("docker.Build: Dockerfile not found: %s", dockerfile)
 	}
 
@@ -150,7 +151,7 @@ func (b *DockerBuilder) Build(ctx context.Context, cfg *build.Config, targets []
 	args = append(args, cfg.ProjectDir)
 
 	// Create output directory
-	if err := os.MkdirAll(cfg.OutputDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(cfg.OutputDir); err != nil {
 		return nil, fmt.Errorf("docker.Build: failed to create output directory: %w", err)
 	}
 

pkg/build/builders/go.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // GoBuilder implements the Builder interface for Go projects.
@@ -27,8 +28,8 @@ func (b *GoBuilder) Name() string {
 
 // Detect checks if this builder can handle the project in the given directory.
 // Uses IsGoProject from the build package which checks for go.mod or wails.json.
-func (b *GoBuilder) Detect(dir string) (bool, error) {
-	return build.IsGoProject(dir), nil
+func (b *GoBuilder) Detect(fs io.Medium, dir string) (bool, error) {
+	return build.IsGoProject(fs, dir), nil
 }
 
 // Build compiles the Go project for the specified targets.
@@ -44,7 +45,7 @@ func (b *GoBuilder) Build(ctx context.Context, cfg *build.Config, targets []buil
 	}
 
 	// Ensure output directory exists
-	if err := os.MkdirAll(cfg.OutputDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(cfg.OutputDir); err != nil {
 		return nil, fmt.Errorf("builders.GoBuilder.Build: failed to create output directory: %w", err)
 	}
 
@@ -76,7 +77,7 @@ func (b *GoBuilder) buildTarget(ctx context.Context, cfg *build.Config, target b
 
 	// Create platform-specific output path: output/os_arch/binary
 	platformDir := filepath.Join(cfg.OutputDir, fmt.Sprintf("%s_%s", target.OS, target.Arch))
-	if err := os.MkdirAll(platformDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(platformDir); err != nil {
 		return build.Artifact{}, fmt.Errorf("failed to create platform directory: %w", err)
 	}
 

pkg/build/builders/go_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -44,13 +45,14 @@ func TestGoBuilder_Name_Good(t *testing.T) {
 }
 
 func TestGoBuilder_Detect_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("detects Go project with go.mod", func(t *testing.T) {
 		dir := t.TempDir()
 		err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module test"), 0644)
 		require.NoError(t, err)
 
 		builder := NewGoBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.True(t, detected)
 	})
@@ -61,7 +63,7 @@ func TestGoBuilder_Detect_Good(t *testing.T) {
 		require.NoError(t, err)
 
 		builder := NewGoBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.True(t, detected)
 	})
@@ -73,7 +75,7 @@ func TestGoBuilder_Detect_Good(t *testing.T) {
 		require.NoError(t, err)
 
 		builder := NewGoBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.False(t, detected)
 	})
@@ -82,7 +84,7 @@ func TestGoBuilder_Detect_Good(t *testing.T) {
 		dir := t.TempDir()
 
 		builder := NewGoBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.False(t, detected)
 	})
@@ -99,6 +101,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "testbinary",
@@ -133,6 +136,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "multitest",
@@ -160,6 +164,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "wintest",
@@ -183,6 +188,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "", // Empty name
@@ -209,6 +215,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "ldflagstest",
@@ -230,6 +237,7 @@ func TestGoBuilder_Build_Good(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "nestedtest",
@@ -261,6 +269,7 @@ func TestGoBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  t.TempDir(),
 			Name:       "test",
@@ -279,6 +288,7 @@ func TestGoBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: "/nonexistent/path",
 			OutputDir:  t.TempDir(),
 			Name:       "test",
@@ -309,6 +319,7 @@ func TestGoBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: dir,
 			OutputDir:  t.TempDir(),
 			Name:       "test",
@@ -335,6 +346,7 @@ func TestGoBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "partialtest",
@@ -360,6 +372,7 @@ func TestGoBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewGoBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  t.TempDir(),
 			Name:       "canceltest",

pkg/build/builders/linuxkit.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // LinuxKitBuilder builds LinuxKit images.
@@ -26,14 +27,22 @@ func (b *LinuxKitBuilder) Name() string {
 }
 
 // Detect checks if a linuxkit.yml or .yml config exists in the directory.
-func (b *LinuxKitBuilder) Detect(dir string) (bool, error) {
+func (b *LinuxKitBuilder) Detect(fs io.Medium, dir string) (bool, error) {
 	// Check for linuxkit.yml
-	if _, err := os.Stat(filepath.Join(dir, "linuxkit.yml")); err == nil {
+	if fs.IsFile(filepath.Join(dir, "linuxkit.yml")) {
 		return true, nil
 	}
-	// Check for .core/linuxkit/*.yml
-	if matches, _ := filepath.Glob(filepath.Join(dir, ".core", "linuxkit", "*.yml")); len(matches) > 0 {
-		return true, nil
+	// Check for .core/linuxkit/
+	lkDir := filepath.Join(dir, ".core", "linuxkit")
+	if fs.IsDir(lkDir) {
+		entries, err := fs.List(lkDir)
+		if err == nil {
+			for _, entry := range entries {
+				if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".yml") {
+					return true, nil
+				}
+			}
+		}
 	}
 	return false, nil
 }
@@ -49,13 +58,21 @@ func (b *LinuxKitBuilder) Build(ctx context.Context, cfg *build.Config, targets
 	configPath := cfg.LinuxKitConfig
 	if configPath == "" {
 		// Auto-detect
-		if _, err := os.Stat(filepath.Join(cfg.ProjectDir, "linuxkit.yml")); err == nil {
+		if cfg.FS.IsFile(filepath.Join(cfg.ProjectDir, "linuxkit.yml")) {
 			configPath = filepath.Join(cfg.ProjectDir, "linuxkit.yml")
 		} else {
 			// Look in .core/linuxkit/
-			matches, _ := filepath.Glob(filepath.Join(cfg.ProjectDir, ".core", "linuxkit", "*.yml"))
-			if len(matches) > 0 {
-				configPath = matches[0]
+			lkDir := filepath.Join(cfg.ProjectDir, ".core", "linuxkit")
+			if cfg.FS.IsDir(lkDir) {
+				entries, err := cfg.FS.List(lkDir)
+				if err == nil {
+					for _, entry := range entries {
+						if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".yml") {
+							configPath = filepath.Join(lkDir, entry.Name())
+							break
+						}
+					}
+				}
 			}
 		}
 	}
@@ -65,7 +82,7 @@ func (b *LinuxKitBuilder) Build(ctx context.Context, cfg *build.Config, targets
 	}
 
 	// Validate config file exists
-	if _, err := os.Stat(configPath); err != nil {
+	if !cfg.FS.IsFile(configPath) {
 		return nil, fmt.Errorf("linuxkit.Build: config file not found: %s", configPath)
 	}
 
@@ -80,7 +97,7 @@ func (b *LinuxKitBuilder) Build(ctx context.Context, cfg *build.Config, targets
 	if outputDir == "" {
 		outputDir = filepath.Join(cfg.ProjectDir, "dist")
 	}
-	if err := os.MkdirAll(outputDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(outputDir); err != nil {
 		return nil, fmt.Errorf("linuxkit.Build: failed to create output directory: %w", err)
 	}
 
@@ -125,9 +142,9 @@ func (b *LinuxKitBuilder) Build(ctx context.Context, cfg *build.Config, targets
 			artifactPath := b.getArtifactPath(outputDir, outputName, format)
 
 			// Verify the artifact was created
-			if _, err := os.Stat(artifactPath); err != nil {
+			if !cfg.FS.Exists(artifactPath) {
 				// Try alternate naming conventions
-				artifactPath = b.findArtifact(outputDir, outputName, format)
+				artifactPath = b.findArtifact(cfg.FS, outputDir, outputName, format)
 				if artifactPath == "" {
 					return nil, fmt.Errorf("linuxkit.Build: artifact not found after build: expected %s", b.getArtifactPath(outputDir, outputName, format))
 				}
@@ -175,7 +192,7 @@ func (b *LinuxKitBuilder) getArtifactPath(outputDir, outputName, format string)
 }
 
 // findArtifact searches for the built artifact with various naming conventions.
-func (b *LinuxKitBuilder) findArtifact(outputDir, outputName, format string) string {
+func (b *LinuxKitBuilder) findArtifact(fs io.Medium, outputDir, outputName, format string) string {
 	// LinuxKit can create files with different suffixes
 	extensions := []string{
 		b.getFormatExtension(format),
@@ -185,18 +202,23 @@ func (b *LinuxKitBuilder) findArtifact(outputDir, outputName, format string) str
 
 	for _, ext := range extensions {
 		path := filepath.Join(outputDir, outputName+ext)
-		if _, err := os.Stat(path); err == nil {
+		if fs.Exists(path) {
 			return path
 		}
 	}
 
 	// Try to find any file matching the output name
-	matches, _ := filepath.Glob(filepath.Join(outputDir, outputName+"*"))
-	for _, match := range matches {
-		// Return first match that looks like an image
-		ext := filepath.Ext(match)
-		if ext == ".iso" || ext == ".qcow2" || ext == ".raw" || ext == ".vmdk" || ext == ".vhd" {
-			return match
+	entries, err := fs.List(outputDir)
+	if err == nil {
+		for _, entry := range entries {
+			if strings.HasPrefix(entry.Name(), outputName) {
+				match := filepath.Join(outputDir, entry.Name())
+				// Return first match that looks like an image
+				ext := filepath.Ext(match)
+				if ext == ".iso" || ext == ".qcow2" || ext == ".raw" || ext == ".vmdk" || ext == ".vhd" {
+					return match
+				}
+			}
 		}
 	}
 

pkg/build/builders/taskfile.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // TaskfileBuilder builds projects using Taskfile (https://taskfile.dev/).
@@ -27,7 +28,7 @@ func (b *TaskfileBuilder) Name() string {
 }
 
 // Detect checks if a Taskfile exists in the directory.
-func (b *TaskfileBuilder) Detect(dir string) (bool, error) {
+func (b *TaskfileBuilder) Detect(fs io.Medium, dir string) (bool, error) {
 	// Check for Taskfile.yml, Taskfile.yaml, or Taskfile
 	taskfiles := []string{
 		"Taskfile.yml",
@@ -38,7 +39,7 @@ func (b *TaskfileBuilder) Detect(dir string) (bool, error) {
 	}
 
 	for _, tf := range taskfiles {
-		if _, err := os.Stat(filepath.Join(dir, tf)); err == nil {
+		if fs.IsFile(filepath.Join(dir, tf)) {
 			return true, nil
 		}
 	}
@@ -57,7 +58,7 @@ func (b *TaskfileBuilder) Build(ctx context.Context, cfg *build.Config, targets
 	if outputDir == "" {
 		outputDir = filepath.Join(cfg.ProjectDir, "dist")
 	}
-	if err := os.MkdirAll(outputDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(outputDir); err != nil {
 		return nil, fmt.Errorf("taskfile.Build: failed to create output directory: %w", err)
 	}
 
@@ -70,7 +71,7 @@ func (b *TaskfileBuilder) Build(ctx context.Context, cfg *build.Config, targets
 		}
 
 		// Try to find artifacts in output directory
-		found := b.findArtifacts(outputDir)
+		found := b.findArtifacts(cfg.FS, outputDir)
 		artifacts = append(artifacts, found...)
 	} else {
 		// Run build task for each target
@@ -80,7 +81,7 @@ func (b *TaskfileBuilder) Build(ctx context.Context, cfg *build.Config, targets
 			}
 
 			// Try to find artifacts for this target
-			found := b.findArtifactsForTarget(outputDir, target)
+			found := b.findArtifactsForTarget(cfg.FS, outputDir, target)
 			artifacts = append(artifacts, found...)
 		}
 	}
@@ -147,10 +148,10 @@ func (b *TaskfileBuilder) runTask(ctx context.Context, cfg *build.Config, goos,
 }
 
 // findArtifacts searches for built artifacts in the output directory.
-func (b *TaskfileBuilder) findArtifacts(outputDir string) []build.Artifact {
+func (b *TaskfileBuilder) findArtifacts(fs io.Medium, outputDir string) []build.Artifact {
 	var artifacts []build.Artifact
 
-	entries, err := os.ReadDir(outputDir)
+	entries, err := fs.List(outputDir)
 	if err != nil {
 		return artifacts
 	}
@@ -177,13 +178,13 @@ func (b *TaskfileBuilder) findArtifacts(outputDir string) []build.Artifact {
 }
 
 // findArtifactsForTarget searches for built artifacts for a specific target.
-func (b *TaskfileBuilder) findArtifactsForTarget(outputDir string, target build.Target) []build.Artifact {
+func (b *TaskfileBuilder) findArtifactsForTarget(fs io.Medium, outputDir string, target build.Target) []build.Artifact {
 	var artifacts []build.Artifact
 
 	// 1. Look for platform-specific subdirectory: output/os_arch/
 	platformSubdir := filepath.Join(outputDir, fmt.Sprintf("%s_%s", target.OS, target.Arch))
-	if info, err := os.Stat(platformSubdir); err == nil && info.IsDir() {
-		entries, _ := os.ReadDir(platformSubdir)
+	if fs.IsDir(platformSubdir) {
+		entries, _ := fs.List(platformSubdir)
 		for _, entry := range entries {
 			if entry.IsDir() {
 				// Handle .app bundles on macOS
@@ -219,18 +220,22 @@ func (b *TaskfileBuilder) findArtifactsForTarget(outputDir string, target build.
 	}
 
 	for _, pattern := range patterns {
-		matches, _ := filepath.Glob(filepath.Join(outputDir, pattern))
-		for _, match := range matches {
-			info, err := os.Stat(match)
-			if err != nil || info.IsDir() {
-				continue
-			}
+		entries, _ := fs.List(outputDir)
+		for _, entry := range entries {
+			match := entry.Name()
+			// Simple glob matching
+			if b.matchPattern(match, pattern) {
+				fullPath := filepath.Join(outputDir, match)
+				if fs.IsDir(fullPath) {
+					continue
+				}
 
-			artifacts = append(artifacts, build.Artifact{
-				Path: match,
-				OS:   target.OS,
-				Arch: target.Arch,
-			})
+				artifacts = append(artifacts, build.Artifact{
+					Path: fullPath,
+					OS:   target.OS,
+					Arch: target.Arch,
+				})
+			}
 		}
 
 		if len(artifacts) > 0 {
@@ -241,6 +246,12 @@ func (b *TaskfileBuilder) findArtifactsForTarget(outputDir string, target build.
 	return artifacts
 }
 
+// matchPattern implements a very simple glob matcher for Taskfile artifacts.
+func (b *TaskfileBuilder) matchPattern(name, pattern string) bool {
+	p := strings.ReplaceAll(pattern, "*", "")
+	return strings.Contains(name, p)
+}
+
 // validateTaskCli checks if the task CLI is available.
 func (b *TaskfileBuilder) validateTaskCli() error {
 	// Check PATH first

pkg/build/builders/wails.go

@@ -4,12 +4,12 @@ package builders
 import (
 	"context"
 	"fmt"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 )
 
 // WailsBuilder implements the Builder interface for Wails v3 projects.
@@ -27,8 +27,8 @@ func (b *WailsBuilder) Name() string {
 
 // Detect checks if this builder can handle the project in the given directory.
 // Uses IsWailsProject from the build package which checks for wails.json.
-func (b *WailsBuilder) Detect(dir string) (bool, error) {
-	return build.IsWailsProject(dir), nil
+func (b *WailsBuilder) Detect(fs io.Medium, dir string) (bool, error) {
+	return build.IsWailsProject(fs, dir), nil
 }
 
 // Build compiles the Wails project for the specified targets.
@@ -45,12 +45,12 @@ func (b *WailsBuilder) Build(ctx context.Context, cfg *build.Config, targets []b
 	}
 
 	// Detect Wails version
-	isV3 := b.isWailsV3(cfg.ProjectDir)
+	isV3 := b.isWailsV3(cfg.FS, cfg.ProjectDir)
 
 	if isV3 {
 		// Wails v3 strategy: Delegate to Taskfile
 		taskBuilder := NewTaskfileBuilder()
-		if detected, _ := taskBuilder.Detect(cfg.ProjectDir); detected {
+		if detected, _ := taskBuilder.Detect(cfg.FS, cfg.ProjectDir); detected {
 			return taskBuilder.Build(ctx, cfg, targets)
 		}
 		return nil, fmt.Errorf("wails v3 projects require a Taskfile for building")
@@ -58,7 +58,7 @@ func (b *WailsBuilder) Build(ctx context.Context, cfg *build.Config, targets []b
 
 	// Wails v2 strategy: Use 'wails build'
 	// Ensure output directory exists
-	if err := os.MkdirAll(cfg.OutputDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(cfg.OutputDir); err != nil {
 		return nil, fmt.Errorf("builders.WailsBuilder.Build: failed to create output directory: %w", err)
 	}
 
@@ -78,13 +78,13 @@ func (b *WailsBuilder) Build(ctx context.Context, cfg *build.Config, targets []b
 }
 
 // isWailsV3 checks if the project uses Wails v3 by inspecting go.mod.
-func (b *WailsBuilder) isWailsV3(dir string) bool {
+func (b *WailsBuilder) isWailsV3(fs io.Medium, dir string) bool {
 	goModPath := filepath.Join(dir, "go.mod")
-	data, err := os.ReadFile(goModPath)
+	content, err := fs.Read(goModPath)
 	if err != nil {
 		return false
 	}
-	return strings.Contains(string(data), "github.com/wailsapp/wails/v3")
+	return strings.Contains(content, "github.com/wailsapp/wails/v3")
 }
 
 // buildV2Target compiles for a single target platform using wails (v2).
@@ -123,7 +123,7 @@ func (b *WailsBuilder) buildV2Target(ctx context.Context, cfg *build.Config, tar
 	wailsOutputDir := filepath.Join(cfg.ProjectDir, "build", "bin")
 
 	// Find the artifact in Wails output dir
-	sourcePath, err := b.findArtifact(wailsOutputDir, binaryName, target)
+	sourcePath, err := b.findArtifact(cfg.FS, wailsOutputDir, binaryName, target)
 	if err != nil {
 		return build.Artifact{}, fmt.Errorf("failed to find Wails v2 build artifact: %w", err)
 	}
@@ -131,18 +131,18 @@ func (b *WailsBuilder) buildV2Target(ctx context.Context, cfg *build.Config, tar
 	// Move/Copy to our output dir
 	// Create platform specific dir in our output
 	platformDir := filepath.Join(cfg.OutputDir, fmt.Sprintf("%s_%s", target.OS, target.Arch))
-	if err := os.MkdirAll(platformDir, 0755); err != nil {
+	if err := cfg.FS.EnsureDir(platformDir); err != nil {
 		return build.Artifact{}, fmt.Errorf("failed to create output dir: %w", err)
 	}
 
 	destPath := filepath.Join(platformDir, filepath.Base(sourcePath))
 
-	// Simple copy
-	input, err := os.ReadFile(sourcePath)
+	// Simple copy using the medium
+	content, err := cfg.FS.Read(sourcePath)
 	if err != nil {
 		return build.Artifact{}, err
 	}
-	if err := os.WriteFile(destPath, input, 0755); err != nil {
+	if err := cfg.FS.Write(destPath, content); err != nil {
 		return build.Artifact{}, err
 	}
 
@@ -154,7 +154,7 @@ func (b *WailsBuilder) buildV2Target(ctx context.Context, cfg *build.Config, tar
 }
 
 // findArtifact locates the built artifact based on the target platform.
-func (b *WailsBuilder) findArtifact(platformDir, binaryName string, target build.Target) (string, error) {
+func (b *WailsBuilder) findArtifact(fs io.Medium, platformDir, binaryName string, target build.Target) (string, error) {
 	var candidates []string
 
 	switch target.OS {
@@ -181,13 +181,13 @@ func (b *WailsBuilder) findArtifact(platformDir, binaryName string, target build
 
 	// Try each candidate
 	for _, candidate := range candidates {
-		if fileOrDirExists(candidate) {
+		if fs.Exists(candidate) {
 			return candidate, nil
 		}
 	}
 
 	// If no specific candidate found, try to find any executable or package in the directory
-	entries, err := os.ReadDir(platformDir)
+	entries, err := fs.List(platformDir)
 	if err != nil {
 		return "", fmt.Errorf("failed to read platform directory: %w", err)
 	}
@@ -221,7 +221,7 @@ func (b *WailsBuilder) findArtifact(platformDir, binaryName string, target build
 
 // detectPackageManager detects the frontend package manager based on lock files.
 // Returns "bun", "pnpm", "yarn", or "npm" (default).
-func detectPackageManager(dir string) string {
+func detectPackageManager(fs io.Medium, dir string) string {
 	// Check in priority order: bun, pnpm, yarn, npm
 	lockFiles := []struct {
 		file    string
@@ -234,7 +234,7 @@ func detectPackageManager(dir string) string {
 	}
 
 	for _, lf := range lockFiles {
-		if fileExists(filepath.Join(dir, lf.file)) {
+		if fs.IsFile(filepath.Join(dir, lf.file)) {
 			return lf.manager
 		}
 	}
@@ -243,20 +243,6 @@ func detectPackageManager(dir string) string {
 	return "npm"
 }
 
-// fileExists checks if a file exists and is not a directory.
-func fileExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-	return !info.IsDir()
-}
-
-// fileOrDirExists checks if a file or directory exists.
-func fileOrDirExists(path string) bool {
-	_, err := os.Stat(path)
-	return err == nil
-}
 
 // Ensure WailsBuilder implements the Builder interface.
 var _ build.Builder = (*WailsBuilder)(nil)

pkg/build/builders/wails_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"github.com/host-uk/core/pkg/build"
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -91,6 +92,7 @@ func TestWailsBuilder_Build_Taskfile_Good(t *testing.T) {
 	}
 
 	t.Run("delegates to Taskfile if present", func(t *testing.T) {
+		fs := io.Local
 		projectDir := setupWailsTestProject(t)
 		outputDir := t.TempDir()
 
@@ -107,6 +109,7 @@ tasks:
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         fs,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "testapp",
@@ -136,11 +139,13 @@ func TestWailsBuilder_Build_V2_Good(t *testing.T) {
 	}
 
 	t.Run("builds v2 project", func(t *testing.T) {
+		fs := io.Local
 		projectDir := setupWailsV2TestProject(t)
 		outputDir := t.TempDir()
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         fs,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "testapp",
@@ -158,13 +163,14 @@ func TestWailsBuilder_Build_V2_Good(t *testing.T) {
 }
 
 func TestWailsBuilder_Detect_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("detects Wails project with wails.json", func(t *testing.T) {
 		dir := t.TempDir()
 		err := os.WriteFile(filepath.Join(dir, "wails.json"), []byte("{}"), 0644)
 		require.NoError(t, err)
 
 		builder := NewWailsBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.True(t, detected)
 	})
@@ -175,7 +181,7 @@ func TestWailsBuilder_Detect_Good(t *testing.T) {
 		require.NoError(t, err)
 
 		builder := NewWailsBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.False(t, detected)
 	})
@@ -186,7 +192,7 @@ func TestWailsBuilder_Detect_Good(t *testing.T) {
 		require.NoError(t, err)
 
 		builder := NewWailsBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.False(t, detected)
 	})
@@ -195,19 +201,20 @@ func TestWailsBuilder_Detect_Good(t *testing.T) {
 		dir := t.TempDir()
 
 		builder := NewWailsBuilder()
-		detected, err := builder.Detect(dir)
+		detected, err := builder.Detect(fs, dir)
 		assert.NoError(t, err)
 		assert.False(t, detected)
 	})
 }
 
 func TestDetectPackageManager_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("detects bun from bun.lockb", func(t *testing.T) {
 		dir := t.TempDir()
 		err := os.WriteFile(filepath.Join(dir, "bun.lockb"), []byte(""), 0644)
 		require.NoError(t, err)
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "bun", result)
 	})
 
@@ -216,7 +223,7 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		err := os.WriteFile(filepath.Join(dir, "pnpm-lock.yaml"), []byte(""), 0644)
 		require.NoError(t, err)
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "pnpm", result)
 	})
 
@@ -225,7 +232,7 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		err := os.WriteFile(filepath.Join(dir, "yarn.lock"), []byte(""), 0644)
 		require.NoError(t, err)
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "yarn", result)
 	})
 
@@ -234,14 +241,14 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		err := os.WriteFile(filepath.Join(dir, "package-lock.json"), []byte(""), 0644)
 		require.NoError(t, err)
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "npm", result)
 	})
 
 	t.Run("defaults to npm when no lock file", func(t *testing.T) {
 		dir := t.TempDir()
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "npm", result)
 	})
 
@@ -252,7 +259,7 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "yarn.lock"), []byte(""), 0644))
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "package-lock.json"), []byte(""), 0644))
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "bun", result)
 	})
 
@@ -263,7 +270,7 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "yarn.lock"), []byte(""), 0644))
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "package-lock.json"), []byte(""), 0644))
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "pnpm", result)
 	})
 
@@ -273,7 +280,7 @@ func TestDetectPackageManager_Good(t *testing.T) {
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "yarn.lock"), []byte(""), 0644))
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "package-lock.json"), []byte(""), 0644))
 
-		result := detectPackageManager(dir)
+		result := detectPackageManager(fs, dir)
 		assert.Equal(t, "yarn", result)
 	})
 }
@@ -293,6 +300,7 @@ func TestWailsBuilder_Build_Bad(t *testing.T) {
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  t.TempDir(),
 			Name:       "test",
@@ -321,6 +329,7 @@ func TestWailsBuilder_Build_Good(t *testing.T) {
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  outputDir,
 			Name:       "testapp",
@@ -359,6 +368,7 @@ func TestWailsBuilder_Ugly(t *testing.T) {
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: dir,
 			OutputDir:  t.TempDir(),
 			Name:       "test",
@@ -386,6 +396,7 @@ func TestWailsBuilder_Ugly(t *testing.T) {
 
 		builder := NewWailsBuilder()
 		cfg := &build.Config{
+			FS:         io.Local,
 			ProjectDir: projectDir,
 			OutputDir:  t.TempDir(),
 			Name:       "canceltest",

pkg/build/checksum.go

@@ -6,20 +6,21 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
-	"os"
 	"path/filepath"
+
+	io_interface "github.com/host-uk/core/pkg/io"
 	"sort"
 	"strings"
 )
 
 // Checksum computes SHA256 for an artifact and returns the artifact with the Checksum field filled.
-func Checksum(artifact Artifact) (Artifact, error) {
+func Checksum(fs io_interface.Medium, artifact Artifact) (Artifact, error) {
 	if artifact.Path == "" {
 		return Artifact{}, fmt.Errorf("build.Checksum: artifact path is empty")
 	}
 
 	// Open the file
-	file, err := os.Open(artifact.Path)
+	file, err := fs.Open(artifact.Path)
 	if err != nil {
 		return Artifact{}, fmt.Errorf("build.Checksum: failed to open file: %w", err)
 	}
@@ -43,14 +44,14 @@ func Checksum(artifact Artifact) (Artifact, error) {
 
 // ChecksumAll computes checksums for all artifacts.
 // Returns a slice of artifacts with their Checksum fields filled.
-func ChecksumAll(artifacts []Artifact) ([]Artifact, error) {
+func ChecksumAll(fs io_interface.Medium, artifacts []Artifact) ([]Artifact, error) {
 	if len(artifacts) == 0 {
 		return nil, nil
 	}
 
 	var checksummed []Artifact
 	for _, artifact := range artifacts {
-		cs, err := Checksum(artifact)
+		cs, err := Checksum(fs, artifact)
 		if err != nil {
 			return checksummed, fmt.Errorf("build.ChecksumAll: failed to checksum %s: %w", artifact.Path, err)
 		}
@@ -67,7 +68,7 @@ func ChecksumAll(artifacts []Artifact) ([]Artifact, error) {
 //
 // The artifacts should have their Checksum fields filled (call ChecksumAll first).
 // Filenames are relative to the output directory (just the basename).
-func WriteChecksumFile(artifacts []Artifact, path string) error {
+func WriteChecksumFile(fs io_interface.Medium, artifacts []Artifact, path string) error {
 	if len(artifacts) == 0 {
 		return nil
 	}
@@ -87,14 +88,8 @@ func WriteChecksumFile(artifacts []Artifact, path string) error {
 
 	content := strings.Join(lines, "\n") + "\n"
 
-	// Ensure directory exists
-	dir := filepath.Dir(path)
-	if err := os.MkdirAll(dir, 0755); err != nil {
-		return fmt.Errorf("build.WriteChecksumFile: failed to create directory: %w", err)
-	}
-
-	// Write the file
-	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+	// Write the file using the medium (which handles directory creation in Write)
+	if err := fs.Write(path, content); err != nil {
 		return fmt.Errorf("build.WriteChecksumFile: failed to write file: %w", err)
 	}
 

pkg/build/checksum_test.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -23,6 +24,7 @@ func setupChecksumTestFile(t *testing.T, content string) string {
 }
 
 func TestChecksum_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("computes SHA256 checksum", func(t *testing.T) {
 		// Known SHA256 of "Hello, World!\n"
 		path := setupChecksumTestFile(t, "Hello, World!\n")
@@ -34,7 +36,7 @@ func TestChecksum_Good(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Checksum(artifact)
+		result, err := Checksum(fs, artifact)
 		require.NoError(t, err)
 		assert.Equal(t, expectedChecksum, result.Checksum)
 	})
@@ -48,7 +50,7 @@ func TestChecksum_Good(t *testing.T) {
 			Arch: "arm64",
 		}
 
-		result, err := Checksum(artifact)
+		result, err := Checksum(fs, artifact)
 		require.NoError(t, err)
 
 		assert.Equal(t, path, result.Path)
@@ -62,7 +64,7 @@ func TestChecksum_Good(t *testing.T) {
 
 		artifact := Artifact{Path: path, OS: "linux", Arch: "amd64"}
 
-		result, err := Checksum(artifact)
+		result, err := Checksum(fs, artifact)
 		require.NoError(t, err)
 
 		// SHA256 produces 32 bytes = 64 hex characters
@@ -73,10 +75,10 @@ func TestChecksum_Good(t *testing.T) {
 		path1 := setupChecksumTestFile(t, "content one")
 		path2 := setupChecksumTestFile(t, "content two")
 
-		result1, err := Checksum(Artifact{Path: path1, OS: "linux", Arch: "amd64"})
+		result1, err := Checksum(fs, Artifact{Path: path1, OS: "linux", Arch: "amd64"})
 		require.NoError(t, err)
 
-		result2, err := Checksum(Artifact{Path: path2, OS: "linux", Arch: "amd64"})
+		result2, err := Checksum(fs, Artifact{Path: path2, OS: "linux", Arch: "amd64"})
 		require.NoError(t, err)
 
 		assert.NotEqual(t, result1.Checksum, result2.Checksum)
@@ -87,10 +89,10 @@ func TestChecksum_Good(t *testing.T) {
 		path1 := setupChecksumTestFile(t, content)
 		path2 := setupChecksumTestFile(t, content)
 
-		result1, err := Checksum(Artifact{Path: path1, OS: "linux", Arch: "amd64"})
+		result1, err := Checksum(fs, Artifact{Path: path1, OS: "linux", Arch: "amd64"})
 		require.NoError(t, err)
 
-		result2, err := Checksum(Artifact{Path: path2, OS: "linux", Arch: "amd64"})
+		result2, err := Checksum(fs, Artifact{Path: path2, OS: "linux", Arch: "amd64"})
 		require.NoError(t, err)
 
 		assert.Equal(t, result1.Checksum, result2.Checksum)
@@ -98,6 +100,7 @@ func TestChecksum_Good(t *testing.T) {
 }
 
 func TestChecksum_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("returns error for empty path", func(t *testing.T) {
 		artifact := Artifact{
 			Path: "",
@@ -105,7 +108,7 @@ func TestChecksum_Bad(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Checksum(artifact)
+		result, err := Checksum(fs, artifact)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "artifact path is empty")
 		assert.Empty(t, result.Checksum)
@@ -118,7 +121,7 @@ func TestChecksum_Bad(t *testing.T) {
 			Arch: "amd64",
 		}
 
-		result, err := Checksum(artifact)
+		result, err := Checksum(fs, artifact)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "failed to open file")
 		assert.Empty(t, result.Checksum)
@@ -126,6 +129,7 @@ func TestChecksum_Bad(t *testing.T) {
 }
 
 func TestChecksumAll_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("checksums multiple artifacts", func(t *testing.T) {
 		paths := []string{
 			setupChecksumTestFile(t, "content one"),
@@ -139,7 +143,7 @@ func TestChecksumAll_Good(t *testing.T) {
 			{Path: paths[2], OS: "windows", Arch: "amd64"},
 		}
 
-		results, err := ChecksumAll(artifacts)
+		results, err := ChecksumAll(fs, artifacts)
 		require.NoError(t, err)
 		require.Len(t, results, 3)
 
@@ -152,19 +156,20 @@ func TestChecksumAll_Good(t *testing.T) {
 	})
 
 	t.Run("returns nil for empty slice", func(t *testing.T) {
-		results, err := ChecksumAll([]Artifact{})
+		results, err := ChecksumAll(fs, []Artifact{})
 		assert.NoError(t, err)
 		assert.Nil(t, results)
 	})
 
 	t.Run("returns nil for nil slice", func(t *testing.T) {
-		results, err := ChecksumAll(nil)
+		results, err := ChecksumAll(fs, nil)
 		assert.NoError(t, err)
 		assert.Nil(t, results)
 	})
 }
 
 func TestChecksumAll_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("returns partial results on error", func(t *testing.T) {
 		path := setupChecksumTestFile(t, "valid content")
 
@@ -173,7 +178,7 @@ func TestChecksumAll_Bad(t *testing.T) {
 			{Path: "/nonexistent/file", OS: "linux", Arch: "arm64"}, // This will fail
 		}
 
-		results, err := ChecksumAll(artifacts)
+		results, err := ChecksumAll(fs, artifacts)
 		assert.Error(t, err)
 		// Should have the first successful result
 		assert.Len(t, results, 1)
@@ -182,6 +187,7 @@ func TestChecksumAll_Bad(t *testing.T) {
 }
 
 func TestWriteChecksumFile_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("writes checksum file with correct format", func(t *testing.T) {
 		dir := t.TempDir()
 		checksumPath := filepath.Join(dir, "CHECKSUMS.txt")
@@ -191,7 +197,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 			{Path: "/output/app_darwin_arm64.tar.gz", Checksum: "789xyz000111", OS: "darwin", Arch: "arm64"},
 		}
 
-		err := WriteChecksumFile(artifacts, checksumPath)
+		err := WriteChecksumFile(fs, artifacts, checksumPath)
 		require.NoError(t, err)
 
 		// Read and verify content
@@ -214,7 +220,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 			{Path: "/output/app.tar.gz", Checksum: "abc123", OS: "linux", Arch: "amd64"},
 		}
 
-		err := WriteChecksumFile(artifacts, checksumPath)
+		err := WriteChecksumFile(fs, artifacts, checksumPath)
 		require.NoError(t, err)
 		assert.FileExists(t, checksumPath)
 	})
@@ -223,7 +229,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 		dir := t.TempDir()
 		checksumPath := filepath.Join(dir, "CHECKSUMS.txt")
 
-		err := WriteChecksumFile([]Artifact{}, checksumPath)
+		err := WriteChecksumFile(fs, []Artifact{}, checksumPath)
 		require.NoError(t, err)
 
 		// File should not exist
@@ -235,7 +241,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 		dir := t.TempDir()
 		checksumPath := filepath.Join(dir, "CHECKSUMS.txt")
 
-		err := WriteChecksumFile(nil, checksumPath)
+		err := WriteChecksumFile(fs, nil, checksumPath)
 		require.NoError(t, err)
 	})
 
@@ -247,7 +253,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 			{Path: "/some/deep/nested/path/myapp_linux_amd64.tar.gz", Checksum: "checksum123", OS: "linux", Arch: "amd64"},
 		}
 
-		err := WriteChecksumFile(artifacts, checksumPath)
+		err := WriteChecksumFile(fs, artifacts, checksumPath)
 		require.NoError(t, err)
 
 		content, err := os.ReadFile(checksumPath)
@@ -260,6 +266,7 @@ func TestWriteChecksumFile_Good(t *testing.T) {
 }
 
 func TestWriteChecksumFile_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("returns error for artifact without checksum", func(t *testing.T) {
 		dir := t.TempDir()
 		checksumPath := filepath.Join(dir, "CHECKSUMS.txt")
@@ -268,7 +275,7 @@ func TestWriteChecksumFile_Bad(t *testing.T) {
 			{Path: "/output/app.tar.gz", Checksum: "", OS: "linux", Arch: "amd64"}, // No checksum
 		}
 
-		err := WriteChecksumFile(artifacts, checksumPath)
+		err := WriteChecksumFile(fs, artifacts, checksumPath)
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "has no checksum")
 	})

pkg/build/config.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 
 	"github.com/host-uk/core/pkg/build/signing"
+	"github.com/host-uk/core/pkg/io"
 	"gopkg.in/yaml.v3"
 )
 
@@ -68,10 +69,10 @@ type TargetConfig struct {
 // LoadConfig loads build configuration from the .core/build.yaml file in the given directory.
 // If the config file does not exist, it returns DefaultConfig().
 // Returns an error if the file exists but cannot be parsed.
-func LoadConfig(dir string) (*BuildConfig, error) {
+func LoadConfig(fs io.Medium, dir string) (*BuildConfig, error) {
 	configPath := filepath.Join(dir, ConfigDir, ConfigFileName)
 
-	data, err := os.ReadFile(configPath)
+	content, err := fs.Read(configPath)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return DefaultConfig(), nil
@@ -80,6 +81,7 @@ func LoadConfig(dir string) (*BuildConfig, error) {
 	}
 
 	var cfg BuildConfig
+	data := []byte(content)
 	if err := yaml.Unmarshal(data, &cfg); err != nil {
 		return nil, fmt.Errorf("build.LoadConfig: failed to parse config file: %w", err)
 	}
@@ -153,8 +155,8 @@ func ConfigPath(dir string) string {
 }
 
 // ConfigExists checks if a build config file exists in the given directory.
-func ConfigExists(dir string) bool {
-	return fileExists(ConfigPath(dir))
+func ConfigExists(fs io.Medium, dir string) bool {
+	return fileExists(fs, ConfigPath(dir))
 }
 
 // ToTargets converts TargetConfig slice to Target slice for use with builders.

pkg/build/config_test.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -28,6 +29,7 @@ func setupConfigTestDir(t *testing.T, configContent string) string {
 }
 
 func TestLoadConfig_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("loads valid config", func(t *testing.T) {
 		content := `
 version: 1
@@ -54,7 +56,7 @@ targets:
 `
 		dir := setupConfigTestDir(t, content)
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		require.NoError(t, err)
 		require.NotNil(t, cfg)
 
@@ -77,7 +79,7 @@ targets:
 	t.Run("returns defaults when config file missing", func(t *testing.T) {
 		dir := t.TempDir()
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		require.NoError(t, err)
 		require.NotNil(t, cfg)
 
@@ -98,7 +100,7 @@ project:
 `
 		dir := setupConfigTestDir(t, content)
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		require.NoError(t, err)
 		require.NotNil(t, cfg)
 
@@ -128,7 +130,7 @@ targets:
 `
 		dir := setupConfigTestDir(t, content)
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		require.NoError(t, err)
 		require.NotNil(t, cfg)
 
@@ -141,6 +143,7 @@ targets:
 }
 
 func TestLoadConfig_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("returns error for invalid YAML", func(t *testing.T) {
 		content := `
 version: 1
@@ -149,7 +152,7 @@ project:
 `
 		dir := setupConfigTestDir(t, content)
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		assert.Error(t, err)
 		assert.Nil(t, cfg)
 		assert.Contains(t, err.Error(), "failed to parse config file")
@@ -166,7 +169,7 @@ project:
 		err = os.Mkdir(configPath, 0755)
 		require.NoError(t, err)
 
-		cfg, err := LoadConfig(dir)
+		cfg, err := LoadConfig(fs, dir)
 		assert.Error(t, err)
 		assert.Nil(t, cfg)
 		assert.Contains(t, err.Error(), "failed to read config file")
@@ -217,19 +220,20 @@ func TestConfigPath_Good(t *testing.T) {
 }
 
 func TestConfigExists_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("returns true when config exists", func(t *testing.T) {
 		dir := setupConfigTestDir(t, "version: 1")
-		assert.True(t, ConfigExists(dir))
+		assert.True(t, ConfigExists(fs, dir))
 	})
 
 	t.Run("returns false when config missing", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, ConfigExists(dir))
+		assert.False(t, ConfigExists(fs, dir))
 	})
 
 	t.Run("returns false when .core dir missing", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, ConfigExists(dir))
+		assert.False(t, ConfigExists(fs, dir))
 	})
 }
 
@@ -249,7 +253,7 @@ sign:
 `
 	_ = os.WriteFile(filepath.Join(coreDir, "build.yaml"), []byte(configContent), 0644)
 
-	cfg, err := LoadConfig(tmpDir)
+	cfg, err := LoadConfig(io.Local, tmpDir)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -298,8 +302,9 @@ func TestBuildConfig_ToTargets_Good(t *testing.T) {
 
 // TestLoadConfig_Testdata tests loading from the testdata fixture.
 func TestLoadConfig_Testdata(t *testing.T) {
+	fs := io.Local
 	t.Run("loads config-project fixture", func(t *testing.T) {
-		cfg, err := LoadConfig("testdata/config-project")
+		cfg, err := LoadConfig(fs, "testdata/config-project")
 		require.NoError(t, err)
 		require.NotNil(t, cfg)
 

pkg/build/discovery.go

@@ -1,9 +1,10 @@
 package build
 
 import (
-	"os"
 	"path/filepath"
 	"slices"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // Marker files for project type detection.
@@ -32,12 +33,12 @@ var markers = []projectMarker{
 // Discover detects project types in the given directory by checking for marker files.
 // Returns a slice of detected project types, ordered by priority (most specific first).
 // For example, a Wails project returns [wails, go] since it has both wails.json and go.mod.
-func Discover(dir string) ([]ProjectType, error) {
+func Discover(fs io.Medium, dir string) ([]ProjectType, error) {
 	var detected []ProjectType
 
 	for _, m := range markers {
 		path := filepath.Join(dir, m.file)
-		if fileExists(path) {
+		if fileExists(fs, path) {
 			// Avoid duplicates (shouldn't happen with current markers, but defensive)
 			if !slices.Contains(detected, m.projectType) {
 				detected = append(detected, m.projectType)
@@ -50,8 +51,8 @@ func Discover(dir string) ([]ProjectType, error) {
 
 // PrimaryType returns the most specific project type detected in the directory.
 // Returns empty string if no project type is detected.
-func PrimaryType(dir string) (ProjectType, error) {
-	types, err := Discover(dir)
+func PrimaryType(fs io.Medium, dir string) (ProjectType, error) {
+	types, err := Discover(fs, dir)
 	if err != nil {
 		return "", err
 	}
@@ -62,31 +63,27 @@ func PrimaryType(dir string) (ProjectType, error) {
 }
 
 // IsGoProject checks if the directory contains a Go project (go.mod or wails.json).
-func IsGoProject(dir string) bool {
-	return fileExists(filepath.Join(dir, markerGoMod)) ||
-		fileExists(filepath.Join(dir, markerWails))
+func IsGoProject(fs io.Medium, dir string) bool {
+	return fileExists(fs, filepath.Join(dir, markerGoMod)) ||
+		fileExists(fs, filepath.Join(dir, markerWails))
 }
 
 // IsWailsProject checks if the directory contains a Wails project.
-func IsWailsProject(dir string) bool {
-	return fileExists(filepath.Join(dir, markerWails))
+func IsWailsProject(fs io.Medium, dir string) bool {
+	return fileExists(fs, filepath.Join(dir, markerWails))
 }
 
 // IsNodeProject checks if the directory contains a Node.js project.
-func IsNodeProject(dir string) bool {
-	return fileExists(filepath.Join(dir, markerNodePackage))
+func IsNodeProject(fs io.Medium, dir string) bool {
+	return fileExists(fs, filepath.Join(dir, markerNodePackage))
 }
 
 // IsPHPProject checks if the directory contains a PHP project.
-func IsPHPProject(dir string) bool {
-	return fileExists(filepath.Join(dir, markerComposer))
+func IsPHPProject(fs io.Medium, dir string) bool {
+	return fileExists(fs, filepath.Join(dir, markerComposer))
 }
 
 // fileExists checks if a file exists and is not a directory.
-func fileExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-	return !info.IsDir()
+func fileExists(fs io.Medium, path string) bool {
+	return fs.IsFile(path)
 }

pkg/build/discovery_test.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -22,52 +23,54 @@ func setupTestDir(t *testing.T, markers ...string) string {
 }
 
 func TestDiscover_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("detects Go project", func(t *testing.T) {
 		dir := setupTestDir(t, "go.mod")
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, []ProjectType{ProjectTypeGo}, types)
 	})
 
 	t.Run("detects Wails project with priority over Go", func(t *testing.T) {
 		dir := setupTestDir(t, "wails.json", "go.mod")
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, []ProjectType{ProjectTypeWails, ProjectTypeGo}, types)
 	})
 
 	t.Run("detects Node.js project", func(t *testing.T) {
 		dir := setupTestDir(t, "package.json")
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, []ProjectType{ProjectTypeNode}, types)
 	})
 
 	t.Run("detects PHP project", func(t *testing.T) {
 		dir := setupTestDir(t, "composer.json")
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, []ProjectType{ProjectTypePHP}, types)
 	})
 
 	t.Run("detects multiple project types", func(t *testing.T) {
 		dir := setupTestDir(t, "go.mod", "package.json")
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, []ProjectType{ProjectTypeGo, ProjectTypeNode}, types)
 	})
 
 	t.Run("empty directory returns empty slice", func(t *testing.T) {
 		dir := t.TempDir()
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Empty(t, types)
 	})
 }
 
 func TestDiscover_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("non-existent directory returns empty slice", func(t *testing.T) {
-		types, err := Discover("/non/existent/path")
+		types, err := Discover(fs, "/non/existent/path")
 		assert.NoError(t, err) // os.Stat fails silently in fileExists
 		assert.Empty(t, types)
 	})
@@ -78,85 +81,90 @@ func TestDiscover_Bad(t *testing.T) {
 		err := os.Mkdir(filepath.Join(dir, "go.mod"), 0755)
 		require.NoError(t, err)
 
-		types, err := Discover(dir)
+		types, err := Discover(fs, dir)
 		assert.NoError(t, err)
 		assert.Empty(t, types)
 	})
 }
 
 func TestPrimaryType_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("returns wails for wails project", func(t *testing.T) {
 		dir := setupTestDir(t, "wails.json", "go.mod")
-		primary, err := PrimaryType(dir)
+		primary, err := PrimaryType(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, ProjectTypeWails, primary)
 	})
 
 	t.Run("returns go for go-only project", func(t *testing.T) {
 		dir := setupTestDir(t, "go.mod")
-		primary, err := PrimaryType(dir)
+		primary, err := PrimaryType(fs, dir)
 		assert.NoError(t, err)
 		assert.Equal(t, ProjectTypeGo, primary)
 	})
 
 	t.Run("returns empty string for empty directory", func(t *testing.T) {
 		dir := t.TempDir()
-		primary, err := PrimaryType(dir)
+		primary, err := PrimaryType(fs, dir)
 		assert.NoError(t, err)
 		assert.Empty(t, primary)
 	})
 }
 
 func TestIsGoProject_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("true with go.mod", func(t *testing.T) {
 		dir := setupTestDir(t, "go.mod")
-		assert.True(t, IsGoProject(dir))
+		assert.True(t, IsGoProject(fs, dir))
 	})
 
 	t.Run("true with wails.json", func(t *testing.T) {
 		dir := setupTestDir(t, "wails.json")
-		assert.True(t, IsGoProject(dir))
+		assert.True(t, IsGoProject(fs, dir))
 	})
 
 	t.Run("false without markers", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, IsGoProject(dir))
+		assert.False(t, IsGoProject(fs, dir))
 	})
 }
 
 func TestIsWailsProject_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("true with wails.json", func(t *testing.T) {
 		dir := setupTestDir(t, "wails.json")
-		assert.True(t, IsWailsProject(dir))
+		assert.True(t, IsWailsProject(fs, dir))
 	})
 
 	t.Run("false with only go.mod", func(t *testing.T) {
 		dir := setupTestDir(t, "go.mod")
-		assert.False(t, IsWailsProject(dir))
+		assert.False(t, IsWailsProject(fs, dir))
 	})
 }
 
 func TestIsNodeProject_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("true with package.json", func(t *testing.T) {
 		dir := setupTestDir(t, "package.json")
-		assert.True(t, IsNodeProject(dir))
+		assert.True(t, IsNodeProject(fs, dir))
 	})
 
 	t.Run("false without package.json", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, IsNodeProject(dir))
+		assert.False(t, IsNodeProject(fs, dir))
 	})
 }
 
 func TestIsPHPProject_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("true with composer.json", func(t *testing.T) {
 		dir := setupTestDir(t, "composer.json")
-		assert.True(t, IsPHPProject(dir))
+		assert.True(t, IsPHPProject(fs, dir))
 	})
 
 	t.Run("false without composer.json", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, IsPHPProject(dir))
+		assert.False(t, IsPHPProject(fs, dir))
 	})
 }
 
@@ -166,27 +174,29 @@ func TestTarget_Good(t *testing.T) {
 }
 
 func TestFileExists_Good(t *testing.T) {
+	fs := io.Local
 	t.Run("returns true for existing file", func(t *testing.T) {
 		dir := t.TempDir()
 		path := filepath.Join(dir, "test.txt")
 		err := os.WriteFile(path, []byte("content"), 0644)
 		require.NoError(t, err)
-		assert.True(t, fileExists(path))
+		assert.True(t, fileExists(fs, path))
 	})
 
 	t.Run("returns false for directory", func(t *testing.T) {
 		dir := t.TempDir()
-		assert.False(t, fileExists(dir))
+		assert.False(t, fileExists(fs, dir))
 	})
 
 	t.Run("returns false for non-existent path", func(t *testing.T) {
-		assert.False(t, fileExists("/non/existent/file"))
+		assert.False(t, fileExists(fs, "/non/existent/file"))
 	})
 }
 
 // TestDiscover_Testdata tests discovery using the testdata fixtures.
 // These serve as integration tests with realistic project structures.
 func TestDiscover_Testdata(t *testing.T) {
+	fs := io.Local
 	testdataDir := "testdata"
 
 	tests := []struct {
@@ -205,7 +215,7 @@ func TestDiscover_Testdata(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			dir := filepath.Join(testdataDir, tt.dir)
-			types, err := Discover(dir)
+			types, err := Discover(fs, dir)
 			assert.NoError(t, err)
 			if len(tt.expected) == 0 {
 				assert.Empty(t, types)

pkg/build/signing/codesign.go

@@ -3,9 +3,10 @@ package signing
 import (
 	"context"
 	"fmt"
-	"os"
 	"os/exec"
 	"runtime"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // MacOSSigner signs binaries using macOS codesign.
@@ -39,7 +40,7 @@ func (s *MacOSSigner) Available() bool {
 }
 
 // Sign codesigns a binary with hardened runtime.
-func (s *MacOSSigner) Sign(ctx context.Context, binary string) error {
+func (s *MacOSSigner) Sign(ctx context.Context, fs io.Medium, binary string) error {
 	if !s.Available() {
 		return fmt.Errorf("codesign.Sign: codesign not available")
 	}
@@ -62,7 +63,7 @@ func (s *MacOSSigner) Sign(ctx context.Context, binary string) error {
 
 // Notarize submits binary to Apple for notarization and staples the ticket.
 // This blocks until Apple responds (typically 1-5 minutes).
-func (s *MacOSSigner) Notarize(ctx context.Context, binary string) error {
+func (s *MacOSSigner) Notarize(ctx context.Context, fs io.Medium, binary string) error {
 	if s.config.AppleID == "" || s.config.TeamID == "" || s.config.AppPassword == "" {
 		return fmt.Errorf("codesign.Notarize: missing Apple credentials (apple_id, team_id, app_password)")
 	}
@@ -73,7 +74,7 @@ func (s *MacOSSigner) Notarize(ctx context.Context, binary string) error {
 	if output, err := zipCmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("codesign.Notarize: failed to create zip: %w\nOutput: %s", err, string(output))
 	}
-	defer func() { _ = os.Remove(zipPath) }()
+	defer func() { _ = fs.Delete(zipPath) }()
 
 	// Submit to Apple and wait
 	submitCmd := exec.CommandContext(ctx, "xcrun", "notarytool", "submit",

pkg/build/signing/codesign_test.go

@@ -5,6 +5,7 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -34,17 +35,19 @@ func TestMacOSSigner_Sign_Bad(t *testing.T) {
 		if runtime.GOOS == "darwin" {
 			t.Skip("skipping on macOS")
 		}
+		fs := io.Local
 		s := NewMacOSSigner(MacOSConfig{Identity: "test"})
-		err := s.Sign(context.Background(), "test")
+		err := s.Sign(context.Background(), fs, "test")
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "not available")
 	})
 }
 
 func TestMacOSSigner_Notarize_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("fails with missing credentials", func(t *testing.T) {
 		s := NewMacOSSigner(MacOSConfig{})
-		err := s.Notarize(context.Background(), "test")
+		err := s.Notarize(context.Background(), fs, "test")
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "missing Apple credentials")
 	})

pkg/build/signing/gpg.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"fmt"
 	"os/exec"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // GPGSigner signs files using GPG.
@@ -35,7 +37,7 @@ func (s *GPGSigner) Available() bool {
 
 // Sign creates a detached ASCII-armored signature.
 // For file.txt, creates file.txt.asc
-func (s *GPGSigner) Sign(ctx context.Context, file string) error {
+func (s *GPGSigner) Sign(ctx context.Context, fs io.Medium, file string) error {
 	if !s.Available() {
 		return fmt.Errorf("gpg.Sign: gpg not available or key not configured")
 	}

pkg/build/signing/gpg_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -23,9 +24,10 @@ func TestGPGSigner_Bad_NoKey(t *testing.T) {
 }
 
 func TestGPGSigner_Sign_Bad(t *testing.T) {
+	fs := io.Local
 	t.Run("fails when no key", func(t *testing.T) {
 		s := NewGPGSigner("")
-		err := s.Sign(context.Background(), "test.txt")
+		err := s.Sign(context.Background(), fs, "test.txt")
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "not available or key not configured")
 	})

pkg/build/signing/sign.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"fmt"
 	"runtime"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // Artifact represents a build output that can be signed.
@@ -16,7 +18,7 @@ type Artifact struct {
 
 // SignBinaries signs macOS binaries in the artifacts list.
 // Only signs darwin binaries when running on macOS with a configured identity.
-func SignBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact) error {
+func SignBinaries(ctx context.Context, fs io.Medium, cfg SignConfig, artifacts []Artifact) error {
 	if !cfg.Enabled {
 		return nil
 	}
@@ -37,7 +39,7 @@ func SignBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact) err
 		}
 
 		fmt.Printf("  Signing %s...\n", artifact.Path)
-		if err := signer.Sign(ctx, artifact.Path); err != nil {
+		if err := signer.Sign(ctx, fs, artifact.Path); err != nil {
 			return fmt.Errorf("failed to sign %s: %w", artifact.Path, err)
 		}
 	}
@@ -46,7 +48,7 @@ func SignBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact) err
 }
 
 // NotarizeBinaries notarizes macOS binaries if enabled.
-func NotarizeBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact) error {
+func NotarizeBinaries(ctx context.Context, fs io.Medium, cfg SignConfig, artifacts []Artifact) error {
 	if !cfg.Enabled || !cfg.MacOS.Notarize {
 		return nil
 	}
@@ -66,7 +68,7 @@ func NotarizeBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact)
 		}
 
 		fmt.Printf("  Notarizing %s (this may take a few minutes)...\n", artifact.Path)
-		if err := signer.Notarize(ctx, artifact.Path); err != nil {
+		if err := signer.Notarize(ctx, fs, artifact.Path); err != nil {
 			return fmt.Errorf("failed to notarize %s: %w", artifact.Path, err)
 		}
 	}
@@ -75,7 +77,7 @@ func NotarizeBinaries(ctx context.Context, cfg SignConfig, artifacts []Artifact)
 }
 
 // SignChecksums signs the checksums file with GPG.
-func SignChecksums(ctx context.Context, cfg SignConfig, checksumFile string) error {
+func SignChecksums(ctx context.Context, fs io.Medium, cfg SignConfig, checksumFile string) error {
 	if !cfg.Enabled {
 		return nil
 	}
@@ -86,7 +88,7 @@ func SignChecksums(ctx context.Context, cfg SignConfig, checksumFile string) err
 	}
 
 	fmt.Printf("  Signing %s with GPG...\n", checksumFile)
-	if err := signer.Sign(ctx, checksumFile); err != nil {
+	if err := signer.Sign(ctx, fs, checksumFile); err != nil {
 		return fmt.Errorf("failed to sign checksums: %w", err)
 	}
 

pkg/build/signing/signer.go

@@ -5,6 +5,8 @@ import (
 	"context"
 	"os"
 	"strings"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // Signer defines the interface for code signing implementations.
@@ -14,7 +16,7 @@ type Signer interface {
 	// Available checks if this signer can be used.
 	Available() bool
 	// Sign signs the artifact at the given path.
-	Sign(ctx context.Context, path string) error
+	Sign(ctx context.Context, fs io.Medium, path string) error
 }
 
 // SignConfig holds signing configuration from .core/build.yaml.

pkg/build/signing/signing_test.go

@@ -5,11 +5,13 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/host-uk/core/pkg/io"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestSignBinaries_Good_SkipsNonDarwin(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: true,
 		MacOS: MacOSConfig{
@@ -23,7 +25,7 @@ func TestSignBinaries_Good_SkipsNonDarwin(t *testing.T) {
 	}
 
 	// Should not error even though binary doesn't exist (skips non-darwin)
-	err := SignBinaries(ctx, cfg, artifacts)
+	err := SignBinaries(ctx, fs, cfg, artifacts)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -31,6 +33,7 @@ func TestSignBinaries_Good_SkipsNonDarwin(t *testing.T) {
 
 func TestSignBinaries_Good_DisabledConfig(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: false,
 	}
@@ -39,7 +42,7 @@ func TestSignBinaries_Good_DisabledConfig(t *testing.T) {
 		{Path: "/tmp/test-binary", OS: "darwin", Arch: "arm64"},
 	}
 
-	err := SignBinaries(ctx, cfg, artifacts)
+	err := SignBinaries(ctx, fs, cfg, artifacts)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -51,6 +54,7 @@ func TestSignBinaries_Good_SkipsOnNonMacOS(t *testing.T) {
 	}
 
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: true,
 		MacOS: MacOSConfig{
@@ -62,7 +66,7 @@ func TestSignBinaries_Good_SkipsOnNonMacOS(t *testing.T) {
 		{Path: "/tmp/test-binary", OS: "darwin", Arch: "arm64"},
 	}
 
-	err := SignBinaries(ctx, cfg, artifacts)
+	err := SignBinaries(ctx, fs, cfg, artifacts)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -70,6 +74,7 @@ func TestSignBinaries_Good_SkipsOnNonMacOS(t *testing.T) {
 
 func TestNotarizeBinaries_Good_DisabledConfig(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: false,
 	}
@@ -78,7 +83,7 @@ func TestNotarizeBinaries_Good_DisabledConfig(t *testing.T) {
 		{Path: "/tmp/test-binary", OS: "darwin", Arch: "arm64"},
 	}
 
-	err := NotarizeBinaries(ctx, cfg, artifacts)
+	err := NotarizeBinaries(ctx, fs, cfg, artifacts)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -86,6 +91,7 @@ func TestNotarizeBinaries_Good_DisabledConfig(t *testing.T) {
 
 func TestNotarizeBinaries_Good_NotarizeDisabled(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: true,
 		MacOS: MacOSConfig{
@@ -97,7 +103,7 @@ func TestNotarizeBinaries_Good_NotarizeDisabled(t *testing.T) {
 		{Path: "/tmp/test-binary", OS: "darwin", Arch: "arm64"},
 	}
 
-	err := NotarizeBinaries(ctx, cfg, artifacts)
+	err := NotarizeBinaries(ctx, fs, cfg, artifacts)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -105,6 +111,7 @@ func TestNotarizeBinaries_Good_NotarizeDisabled(t *testing.T) {
 
 func TestSignChecksums_Good_SkipsNoKey(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: true,
 		GPG: GPGConfig{
@@ -113,7 +120,7 @@ func TestSignChecksums_Good_SkipsNoKey(t *testing.T) {
 	}
 
 	// Should silently skip when no key
-	err := SignChecksums(ctx, cfg, "/tmp/CHECKSUMS.txt")
+	err := SignChecksums(ctx, fs, cfg, "/tmp/CHECKSUMS.txt")
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -121,11 +128,12 @@ func TestSignChecksums_Good_SkipsNoKey(t *testing.T) {
 
 func TestSignChecksums_Good_Disabled(t *testing.T) {
 	ctx := context.Background()
+	fs := io.Local
 	cfg := SignConfig{
 		Enabled: false,
 	}
 
-	err := SignChecksums(ctx, cfg, "/tmp/CHECKSUMS.txt")
+	err := SignChecksums(ctx, fs, cfg, "/tmp/CHECKSUMS.txt")
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -146,8 +154,9 @@ func TestSignConfig_ExpandEnv(t *testing.T) {
 }
 
 func TestWindowsSigner_Good(t *testing.T) {
+	fs := io.Local
 	s := NewWindowsSigner(WindowsConfig{})
 	assert.Equal(t, "signtool", s.Name())
 	assert.False(t, s.Available())
-	assert.NoError(t, s.Sign(context.Background(), "test.exe"))
+	assert.NoError(t, s.Sign(context.Background(), fs, "test.exe"))
 }

pkg/build/signing/signtool.go

@@ -2,6 +2,8 @@ package signing
 
 import (
 	"context"
+
+	"github.com/host-uk/core/pkg/io"
 )
 
 // WindowsSigner signs binaries using Windows signtool (placeholder).
@@ -28,7 +30,7 @@ func (s *WindowsSigner) Available() bool {
 }
 
 // Sign is a placeholder that does nothing.
-func (s *WindowsSigner) Sign(ctx context.Context, binary string) error {
+func (s *WindowsSigner) Sign(ctx context.Context, fs io.Medium, binary string) error {
 	// TODO: Implement Windows signing
 	return nil
 }

pkg/io/io.go

@@ -1,6 +1,7 @@
 package io
 
 import (
+	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -48,6 +49,12 @@ type Medium interface {
 	// Stat returns file information for the given path.
 	Stat(path string) (fs.FileInfo, error)
 
+	// Open opens the named file for reading.
+	Open(path string) (fs.File, error)
+
+	// Create creates or truncates the named file.
+	Create(path string) (io.WriteCloser, error)
+
 	// Exists checks if a path exists (file or directory).
 	Exists(path string) bool
 
@@ -307,6 +314,70 @@ func (m *MockMedium) Rename(oldPath, newPath string) error {
 	return coreerr.E("io.MockMedium.Rename", "path not found: "+oldPath, os.ErrNotExist)
 }
 
+// Open opens a file from the mock filesystem.
+func (m *MockMedium) Open(path string) (fs.File, error) {
+	content, ok := m.Files[path]
+	if !ok {
+		return nil, coreerr.E("io.MockMedium.Open", "file not found: "+path, os.ErrNotExist)
+	}
+	return &MockFile{
+		name:    filepath.Base(path),
+		content: []byte(content),
+	}, nil
+}
+
+// Create creates a file in the mock filesystem.
+func (m *MockMedium) Create(path string) (io.WriteCloser, error) {
+	return &MockWriteCloser{
+		medium: m,
+		path:   path,
+	}, nil
+}
+
+// MockFile implements fs.File for MockMedium.
+type MockFile struct {
+	name    string
+	content []byte
+	offset  int64
+}
+
+func (f *MockFile) Stat() (fs.FileInfo, error) {
+	return FileInfo{
+		name: f.name,
+		size: int64(len(f.content)),
+	}, nil
+}
+
+func (f *MockFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.content)) {
+		return 0, fs.ErrClosed // Or io.EOF?
+	}
+	n := copy(b, f.content[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *MockFile) Close() error {
+	return nil
+}
+
+// MockWriteCloser implements WriteCloser for MockMedium.
+type MockWriteCloser struct {
+	medium *MockMedium
+	path   string
+	data   []byte
+}
+
+func (w *MockWriteCloser) Write(p []byte) (int, error) {
+	w.data = append(w.data, p...)
+	return len(p), nil
+}
+
+func (w *MockWriteCloser) Close() error {
+	w.medium.Files[w.path] = string(w.data)
+	return nil
+}
+
 // List returns directory entries for the mock filesystem.
 func (m *MockMedium) List(path string) ([]fs.DirEntry, error) {
 	if _, ok := m.Dirs[path]; !ok {

pkg/io/local/client.go

@@ -2,6 +2,7 @@
 package local
 
 import (
+	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -106,6 +107,20 @@ func (m *Medium) Stat(p string) (fs.FileInfo, error) {
 	return os.Stat(m.path(p))
 }
 
+// Open opens the named file for reading.
+func (m *Medium) Open(p string) (fs.File, error) {
+	return os.Open(m.path(p))
+}
+
+// Create creates or truncates the named file.
+func (m *Medium) Create(p string) (io.WriteCloser, error) {
+	full := m.path(p)
+	if err := os.MkdirAll(filepath.Dir(full), 0755); err != nil {
+		return nil, err
+	}
+	return os.Create(full)
+}
+
 // Delete removes a file or empty directory.
 func (m *Medium) Delete(p string) error {
 	full := m.path(p)

pkg/release/release.go

@@ -208,8 +208,11 @@ func Run(ctx context.Context, cfg *Config, dryRun bool) (*Release, error) {
 
 // buildArtifacts builds all artifacts for the release.
 func buildArtifacts(ctx context.Context, cfg *Config, projectDir, version string) ([]build.Artifact, error) {
+	// Use local filesystem as the default medium
+	fs := io.Local
+
 	// Load build configuration
-	buildCfg, err := build.LoadConfig(projectDir)
+	buildCfg, err := build.LoadConfig(fs, projectDir)
 	if err != nil {
 		return nil, fmt.Errorf("failed to load build config: %w", err)
 	}
@@ -248,7 +251,7 @@ func buildArtifacts(ctx context.Context, cfg *Config, projectDir, version string
 	outputDir := filepath.Join(projectDir, "dist")
 
 	// Get builder (detect project type)
-	projectType, err := build.PrimaryType(projectDir)
+	projectType, err := build.PrimaryType(fs, projectDir)
 	if err != nil {
 		return nil, fmt.Errorf("failed to detect project type: %w", err)
 	}
@@ -260,6 +263,7 @@ func buildArtifacts(ctx context.Context, cfg *Config, projectDir, version string
 
 	// Build configuration
 	buildConfig := &build.Config{
+		FS:         fs,
 		ProjectDir: projectDir,
 		OutputDir:  outputDir,
 		Name:       binaryName,
@@ -274,20 +278,20 @@ func buildArtifacts(ctx context.Context, cfg *Config, projectDir, version string
 	}
 
 	// Archive artifacts
-	archivedArtifacts, err := build.ArchiveAll(artifacts)
+	archivedArtifacts, err := build.ArchiveAll(fs, artifacts)
 	if err != nil {
 		return nil, fmt.Errorf("archive failed: %w", err)
 	}
 
 	// Compute checksums
-	checksummedArtifacts, err := build.ChecksumAll(archivedArtifacts)
+	checksummedArtifacts, err := build.ChecksumAll(fs, archivedArtifacts)
 	if err != nil {
 		return nil, fmt.Errorf("checksum failed: %w", err)
 	}
 
 	// Write CHECKSUMS.txt
 	checksumPath := filepath.Join(outputDir, "CHECKSUMS.txt")
-	if err := build.WriteChecksumFile(checksummedArtifacts, checksumPath); err != nil {
+	if err := build.WriteChecksumFile(fs, checksummedArtifacts, checksumPath); err != nil {
 		return nil, fmt.Errorf("failed to write checksums file: %w", err)
 	}