feat: gt doctor checks for branch hygiene

Add two new doctor checks:

1. persistent-role-branches: Detects crew/*, witness/rig, refinery/rig
   directories not on main branch. Persistent roles should work directly
   on main to avoid orphaned work.

2. beads-sync-orphans: Detects code changes on beads-sync branch that
   weren't merged to main, catching cases where merges lose code changes.

Also adds ensureMainBranch() to crew attach to auto-switch persistent
roles to main at session start.

Closes gt-p9zh

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

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

internal/cmd/doctor.go

@@ -57,6 +57,8 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewBeadsDatabaseCheck())
 	d.Register(doctor.NewOrphanSessionCheck())
 	d.Register(doctor.NewOrphanProcessCheck())
+	d.Register(doctor.NewBranchCheck())
+	d.Register(doctor.NewBeadsSyncOrphanCheck())
 
 	// Run checks
 	var report *doctor.Report