Link MR bead to agent bead for traceability (gt-84ery)

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>

internal/mrqueue/mrqueue.go

@@ -26,6 +26,7 @@ type MR struct {
 	Title       string    `json:"title"`        // MR title
 	Priority    int       `json:"priority"`     // Priority (lower = higher priority)
 	CreatedAt   time.Time `json:"created_at"`
+	AgentBead   string    `json:"agent_bead,omitempty"` // Agent bead ID that created this MR (for traceability)
 }
 
 // Queue manages the MR storage.