feat(doctor): add hook-singleton check (gt-h6eq.1)

Add doctor check to ensure each agent has at most one handoff bead.
Detects when multiple pinned beads exist with the same "{role} Handoff"
title, which can cause confusion about which handoff is authoritative.

- Check: hook-singleton
- Error if: Multiple pinned beads with same '{role} Handoff' title
- Fix: Automatically closes duplicates (keeps oldest) when run with --fix

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

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

internal/cmd/doctor.go

@@ -101,6 +101,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 
 	// Hook attachment checks
 	d.Register(doctor.NewHookAttachmentValidCheck())
+	d.Register(doctor.NewHookSingletonCheck())
 
 	// Run checks
 	var report *doctor.Report