Implements `gt callbacks process` for Mayor/Deacon patrol to handle
callbacks from agents:
- POLECAT_DONE: Log polecat completion
- MERGE_COMPLETED: Close source issue, log merge
- MERGE_REJECTED: Log rejection reason
- HELP/ESCALATION: Forward to overseer
- SLING_REQUEST: Log sling request for spawn
- WITNESS_REPORT: Log rig health status
- REFINERY_REPORT: Log queue status
Also adds EventCallback to townlog for callback processing events.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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