feat: Implement bare repo architecture for branch visibility (gt-gmqe)

- Add .repo.git as shared bare repo for worktrees
- Update polecat manager to use bare repo when available
- Add git.NewGitWithDir() and CloneBare() for bare repo support
- Update gt rig init to create bare repo architecture for new rigs
- Refinery and polecats now share branch visibility via shared .git

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/cmd/rig.go

@@ -217,10 +217,11 @@ func runRigAdd(cmd *cobra.Command, args []string) error {
 	fmt.Printf("\nStructure:\n")
 	fmt.Printf("  %s/\n", name)
 	fmt.Printf("  ├── config.json\n")
+	fmt.Printf("  ├── .repo.git/        (shared bare repo)\n")
 	fmt.Printf("  ├── .beads/           (prefix: %s)\n", newRig.Config.Prefix)
 	fmt.Printf("  ├── plugins/          (rig-level plugins)\n")
-	fmt.Printf("  ├── refinery/rig/     (canonical main)\n")
-	fmt.Printf("  ├── mayor/rig/        (mayor's clone)\n")
+	fmt.Printf("  ├── mayor/rig/        (worktree: main)\n")
+	fmt.Printf("  ├── refinery/rig/     (worktree: refinery)\n")
 	fmt.Printf("  ├── crew/%s/        (your workspace)\n", crewName)
 	fmt.Printf("  ├── witness/\n")
 	fmt.Printf("  └── polecats/\n")