Add bidirectional cross-references between MR beads and agent beads:
1. MRFields.AgentBead - tracks which agent created the MR
2. AgentFields.ActiveMR - tracks agent's current MR
In gt done:
- Include agent_bead in MR description when creating
- Update agent bead with active_mr pointing to the new MR
In refinery merge handling:
- Clear agent bead's active_mr after successful merge
Benefits:
- Given MR, find which polecat created it
- Given polecat, find their active MR
- Orphan detection: MR without agent = stale
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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