chore(io): Complete migration of pkg/repos to io.Medium

- Migrated pkg/repos/registry.go to use io.Medium abstraction for all file operations.
- Updated all callers in internal/cmd/ to pass io.Local, with proper alias handling.
- Fixed formatting issues in cmd_github.go that caused previous CI failures.
- Added unit tests in pkg/repos/registry_test.go using io.MockMedium.
- Verified project-wide build and new unit tests pass.

pkg/repos/registry_test.go

@@ -0,0 +1,77 @@
+package repos
+
+import (
+	"testing"
+
+	"github.com/host-uk/core/pkg/io"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLoadRegistry(t *testing.T) {
+	m := io.NewMockMedium()
+	yaml := `
+version: 1
+org: host-uk
+base_path: /tmp/repos
+repos:
+  core:
+    type: foundation
+    description: Core package
+`
+	m.Write("/tmp/repos.yaml", yaml)
+
+	reg, err := LoadRegistry(m, "/tmp/repos.yaml")
+	assert.NoError(t, err)
+	assert.NotNil(t, reg)
+	assert.Equal(t, "host-uk", reg.Org)
+	assert.Equal(t, "/tmp/repos", reg.BasePath)
+	assert.Equal(t, m, reg.medium)
+
+	repo, ok := reg.Get("core")
+	assert.True(t, ok)
+	assert.Equal(t, "core", repo.Name)
+	assert.Equal(t, "/tmp/repos/core", repo.Path)
+	assert.Equal(t, reg, repo.registry)
+}
+
+func TestRepo_Exists(t *testing.T) {
+	m := io.NewMockMedium()
+	reg := &Registry{
+		medium:   m,
+		BasePath: "/tmp/repos",
+		Repos:    make(map[string]*Repo),
+	}
+	repo := &Repo{
+		Name:     "core",
+		Path:     "/tmp/repos/core",
+		registry: reg,
+	}
+
+	// Not exists yet
+	assert.False(t, repo.Exists())
+
+	// Create directory in mock
+	m.EnsureDir("/tmp/repos/core")
+	assert.True(t, repo.Exists())
+}
+
+func TestRepo_IsGitRepo(t *testing.T) {
+	m := io.NewMockMedium()
+	reg := &Registry{
+		medium:   m,
+		BasePath: "/tmp/repos",
+		Repos:    make(map[string]*Repo),
+	}
+	repo := &Repo{
+		Name:     "core",
+		Path:     "/tmp/repos/core",
+		registry: reg,
+	}
+
+	// Not a git repo yet
+	assert.False(t, repo.IsGitRepo())
+
+	// Create .git directory in mock
+	m.EnsureDir("/tmp/repos/core/.git")
+	assert.True(t, repo.IsGitRepo())
+}