Enable parallel checkout for faster worktree creation

Add EnableParallelCheckout() helper to git package and call it when
creating bare repos. This configures checkout.workers=0 (auto-detect
cores) and checkout.thresholdForParallelism=100.

Expected 2-8x speedup on SSD for large repos (e.g., java rig with 85k files).
Based on research in hq-dqc7t (APFS CoW optimization).

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

internal/rig/manager.go

@@ -13,8 +13,8 @@ import (
 
 	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/claude"
-	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/config"
+	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/git"
 )
 
@@ -342,6 +342,13 @@ func (m *Manager) AddRig(opts AddRigOptions) (*Rig, error) {
 	fmt.Printf("   ✓ Created shared bare repo\n")
 	bareGit := git.NewGitWithDir(bareRepoPath, "")
 
+	// Enable parallel checkout for faster worktree creation on large repos.
+	// This is especially beneficial for repos with 10k+ files (e.g., java rig).
+	if err := bareGit.EnableParallelCheckout(); err != nil {
+		// Non-fatal: parallel checkout is an optimization, not required
+		fmt.Printf("   Warning: could not enable parallel checkout: %v\n", err)
+	}
+
 	// Determine default branch: use provided value or auto-detect from remote
 	var defaultBranch string
 	if opts.DefaultBranch != "" {