Add merge queue activity events to the refinery:
- merge_started: When refinery begins processing an MR
- merged: When MR successfully merged to main
- merge_failed: When merge fails (conflict, tests, push, etc.)
- merge_skipped: When MR skipped (superseded)
Events include MR ID, worker, branch, and reason (for failures).
Implemented in both Manager.ProcessMR and Engineer.ProcessMRFromQueue.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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