Add bd doctor check that detects legacy gastown merge queue JSON files
in .beads/mq/. These files are local-only remnants from the old mrqueue
implementation and can safely be deleted since gt done already creates
merge-request wisps in beads.
- CheckStaleMQFiles() detects .beads/mq/*.json files
- FixStaleMQFiles() removes the entire mq directory
- Comprehensive tests for check and fix
This is the first step toward removing the mrqueue side-channel from
gastown. The follow-up convoy will update Refinery/Witness to use
beads exclusively.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>