feat(devops): add GitHub Releases source

Downloads core-devops images from GitHub Releases using gh CLI.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/devops/sources/github.go

@@ -0,0 +1,70 @@
+package sources
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// GitHubSource downloads images from GitHub Releases.
+type GitHubSource struct {
+	config SourceConfig
+}
+
+// Compile-time interface check.
+var _ ImageSource = (*GitHubSource)(nil)
+
+// NewGitHubSource creates a new GitHub source.
+func NewGitHubSource(cfg SourceConfig) *GitHubSource {
+	return &GitHubSource{config: cfg}
+}
+
+// Name returns "github".
+func (s *GitHubSource) Name() string {
+	return "github"
+}
+
+// Available checks if gh CLI is installed and authenticated.
+func (s *GitHubSource) Available() bool {
+	_, err := exec.LookPath("gh")
+	if err != nil {
+		return false
+	}
+	// Check if authenticated
+	cmd := exec.Command("gh", "auth", "status")
+	return cmd.Run() == nil
+}
+
+// LatestVersion returns the latest release tag.
+func (s *GitHubSource) LatestVersion(ctx context.Context) (string, error) {
+	cmd := exec.CommandContext(ctx, "gh", "release", "view",
+		"-R", s.config.GitHubRepo,
+		"--json", "tagName",
+		"-q", ".tagName",
+	)
+	out, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("github.LatestVersion: %w", err)
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+// Download downloads the image from the latest release.
+func (s *GitHubSource) Download(ctx context.Context, dest string, progress func(downloaded, total int64)) error {
+	// Get release assets to find our image
+	cmd := exec.CommandContext(ctx, "gh", "release", "download",
+		"-R", s.config.GitHubRepo,
+		"-p", s.config.ImageName,
+		"-D", dest,
+		"--clobber",
+	)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("github.Download: %w", err)
+	}
+	return nil
+}

pkg/devops/sources/github_test.go

@@ -0,0 +1,19 @@
+package sources
+
+import (
+	"testing"
+)
+
+func TestGitHubSource_Good_Available(t *testing.T) {
+	src := NewGitHubSource(SourceConfig{
+		GitHubRepo: "host-uk/core-images",
+		ImageName:  "core-devops-darwin-arm64.qcow2",
+	})
+
+	if src.Name() != "github" {
+		t.Errorf("expected name 'github', got %q", src.Name())
+	}
+
+	// Available depends on gh CLI being installed
+	_ = src.Available()
+}