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