feat(platform): add GitClient helper with idempotent EnsureRepo

internal/platform/git.go

@@ -0,0 +1,23 @@
+package platform
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+)
+
+// GitClient wraps `git` via a Runner.
+type GitClient struct{ Runner Runner }
+
+// Clone always runs `git clone --depth=1 url dest`.
+func (g GitClient) Clone(ctx context.Context, url, dest string) error {
+	return g.Runner.Run(ctx, "git", "clone", "--depth=1", url, dest)
+}
+
+// EnsureRepo clones url into dest if dest/.git is missing; otherwise noop.
+func (g GitClient) EnsureRepo(ctx context.Context, url, dest string) error {
+	if _, err := os.Stat(filepath.Join(dest, ".git")); err == nil {
+		return nil
+	}
+	return g.Clone(ctx, url, dest)
+}