fix(skills-init): skip existing Git skills

go/core/internal/skillsinit/git.go

@@ -10,6 +10,41 @@ import (
 
 var immutableGitCommit = regexp.MustCompile(`^([0-9a-fA-F]{40}|[0-9a-fA-F]{64})$`)
 
+// existingGitSkill reports whether destination already contains a materialized
+// skill. A destination without a regular SKILL.md is not safe to treat as a
+// completed download, because Run would otherwise continue with an unusable
+// skill after a partial initialization.
+func existingGitSkill(destination string) (bool, error) {
+	info, err := os.Lstat(destination)
+	if os.IsNotExist(err) {
+		return false, nil
+	}
+	if err != nil {
+		return false, fmt.Errorf("inspect git destination: %w", err)
+	}
+	if info.Mode()&os.ModeSymlink != 0 {
+		return false, fmt.Errorf("existing git destination is not a skill: destination is a symbolic link")
+	}
+	if !info.IsDir() {
+		return false, fmt.Errorf("existing git destination is not a skill: destination is not a directory")
+	}
+
+	skill, err := os.Lstat(filepath.Join(destination, "SKILL.md"))
+	if os.IsNotExist(err) {
+		return false, fmt.Errorf("existing git destination is not a skill: SKILL.md is required")
+	}
+	if err != nil {
+		return false, fmt.Errorf("inspect existing git skill: %w", err)
+	}
+	if skill.Mode()&os.ModeSymlink != 0 {
+		return false, fmt.Errorf("existing git destination is not a skill: SKILL.md is a symbolic link")
+	}
+	if !skill.Mode().IsRegular() {
+		return false, fmt.Errorf("existing git destination is not a skill: SKILL.md is not a regular file")
+	}
+	return true, nil
+}
+
 // CloneGit fetches a single git ref into ref.Dest. All user-controlled
 // strings (URL, Ref, SubPath) are passed to git as separate argv entries via
 // exec.Command — they never pass through a shell, so metacharacters in any of
@@ -22,6 +57,14 @@ var immutableGitCommit = regexp.MustCompile(`^([0-9a-fA-F]{40}|[0-9a-fA-F]{64})$
 // SubPath, if set, rewrites the destination so the final layout matches the
 // requested in-repo subdirectory.
 func CloneGit(ref GitRef) error {
+	exists, err := existingGitSkill(ref.Dest)
+	if err != nil {
+		return err
+	}
+	if exists {
+		return nil
+	}
+
 	if ref.Full {
 		if err := runGit("clone", "--", ref.URL, ref.Dest); err != nil {
 			return err

go/core/internal/skillsinit/git_test.go

@@ -47,3 +47,56 @@ func TestCloneGitCommitRejectsMutableRef(t *testing.T) {
 	err := CloneGitCommit("https://example.com/repository.git", "main", t.TempDir())
 	require.ErrorContains(t, err, "full SHA")
 }
+
+func TestCloneGitSkipsExistingDestination(t *testing.T) {
+	dest := filepath.Join(t.TempDir(), "existing-skill")
+	require.NoError(t, os.MkdirAll(dest, 0o755))
+	existingSkill := filepath.Join(dest, "SKILL.md")
+	require.NoError(t, os.WriteFile(existingSkill, []byte("existing skill\n"), 0o644))
+
+	err := CloneGit(GitRef{
+		URL:  filepath.Join(t.TempDir(), "unreachable-source"),
+		Ref:  "main",
+		Dest: dest,
+	})
+
+	require.NoError(t, err)
+	content, err := os.ReadFile(existingSkill)
+	require.NoError(t, err)
+	assert.Equal(t, "existing skill\n", string(content))
+}
+
+func TestCloneGitRejectsExistingDestinationWithoutSkill(t *testing.T) {
+	dest := filepath.Join(t.TempDir(), "incomplete-skill")
+	require.NoError(t, os.MkdirAll(dest, 0o755))
+
+	err := CloneGit(GitRef{
+		URL:  filepath.Join(t.TempDir(), "unreachable-source"),
+		Ref:  "main",
+		Dest: dest,
+	})
+
+	require.ErrorContains(t, err, "existing git destination is not a skill")
+}
+
+func TestExistingGitSkillRejectsSymlinks(t *testing.T) {
+	t.Run("destination", func(t *testing.T) {
+		target := t.TempDir()
+		require.NoError(t, os.WriteFile(filepath.Join(target, "SKILL.md"), []byte("skill\n"), 0o644))
+		destination := filepath.Join(t.TempDir(), "skill")
+		require.NoError(t, os.Symlink(target, destination))
+
+		_, err := existingGitSkill(destination)
+		require.ErrorContains(t, err, "symbolic link")
+	})
+
+	t.Run("skill file", func(t *testing.T) {
+		destination := t.TempDir()
+		target := filepath.Join(t.TempDir(), "SKILL.md")
+		require.NoError(t, os.WriteFile(target, []byte("skill\n"), 0o644))
+		require.NoError(t, os.Symlink(target, filepath.Join(destination, "SKILL.md")))
+
+		_, err := existingGitSkill(destination)
+		require.ErrorContains(t, err, "symbolic link")
+	})
+}