perf(git): enable parallel checkout for faster worktree creation

Configure checkout.workers=0 (auto-detect CPU count) on all clone
operations. Git 2.33+ supports this feature which parallelizes file
creation during checkout, significantly speeding up worktree creation.

This is applied to:
- Clone() - regular clones
- CloneWithReference() - clones with local reference
- CloneBare() - bare clones (primary source for worktrees)
- CloneBareWithReference() - bare clones with reference

The config is set per-repo so worktrees inherit it automatically.
Older git versions silently ignore this config (non-fatal).

Recovered from: 4ed3e983 (lost commit)

internal/git/git.go

@@ -266,6 +266,8 @@ func (g *Git) Clone(url, dest string) error {
 	if err := configureHooksPath(dest); err != nil {
 		return err
 	}
+	// Enable parallel checkout for faster worktree operations
+	_ = configureParallelCheckout(dest)
 	// Configure sparse checkout to exclude .claude/ from source repo
 	return ConfigureSparseCheckout(dest)
 }
@@ -306,6 +308,8 @@ func (g *Git) CloneWithReference(url, dest, reference string) error {
 	if err := configureHooksPath(dest); err != nil {
 		return err
 	}
+	// Enable parallel checkout for faster worktree operations
+	_ = configureParallelCheckout(dest)
 	// Configure sparse checkout to exclude .claude/ from source repo
 	return ConfigureSparseCheckout(dest)
 }
@@ -342,10 +346,27 @@ func (g *Git) CloneBare(url, dest string) error {
 		return fmt.Errorf("moving clone to destination: %w", err)
 	}
 
+	// Enable parallel checkout for faster worktree operations
+	_ = configureParallelCheckout(dest)
 	// Configure refspec so worktrees can fetch and see origin/* refs
 	return configureRefspec(dest)
 }
 
+// configureParallelCheckout enables parallel checkout workers for faster file operations.
+// Git 2.33+ supports checkout.workers which parallelizes file creation during checkout,
+// significantly speeding up worktree creation and clone operations.
+func configureParallelCheckout(repoPath string) error {
+	// Enable parallel checkout with automatic worker count (uses all CPUs)
+	cmd := exec.Command("git", "-C", repoPath, "config", "checkout.workers", "0")
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		// Non-fatal: older git versions don't support this
+		return nil
+	}
+	return nil
+}
+
 // configureHooksPath sets core.hooksPath to use the repo's .githooks directory
 // if it exists. This ensures Gas Town agents use the pre-push hook that blocks
 // pushes to non-main branches (internal PRs are not allowed).
@@ -424,6 +445,8 @@ func (g *Git) CloneBareWithReference(url, dest, reference string) error {
 		return fmt.Errorf("moving clone to destination: %w", err)
 	}
 
+	// Enable parallel checkout for faster worktree operations
+	_ = configureParallelCheckout(dest)
 	// Configure refspec so worktrees can fetch and see origin/* refs
 	return configureRefspec(dest)
 }
@@ -483,10 +506,10 @@ func (g *Git) CommitAll(message string) error {
 
 // GitStatus represents the status of the working directory.
 type GitStatus struct {
-	Clean    bool
-	Modified []string
-	Added    []string
-	Deleted  []string
+	Clean     bool
+	Modified  []string
+	Added     []string
+	Deleted   []string
 	Untracked []string
 }
 
@@ -1208,8 +1231,8 @@ type UncommittedWorkStatus struct {
 	StashCount            int
 	UnpushedCommits       int
 	// Details for error messages
-	ModifiedFiles   []string
-	UntrackedFiles  []string
+	ModifiedFiles  []string
+	UntrackedFiles []string
 }
 
 // Clean returns true if there is no uncommitted work.