fix: auto-configure sync-branch for rig beads

New rigs now get sync-branch: beads-sync in their .beads/config.yaml
automatically. This enables multi-clone coordination for polecats,
crew members, and refinery.

Also added gt doctor check (beads-sync-branch) to verify existing rigs
have sync-branch configured, with --fix support.

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

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

internal/cmd/doctor.go

@@ -55,6 +55,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Register built-in checks
 	d.Register(doctor.NewDaemonCheck())
 	d.Register(doctor.NewBeadsDatabaseCheck())
+	d.Register(doctor.NewBeadsSyncBranchCheck())
 
 	// Run checks
 	var report *doctor.Report