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/cmd/done.go

@@ -108,6 +108,19 @@ func runDone(cmd *cobra.Command, args []string) error {
 		polecatName = parts[len(parts)-1]
 	}
 
+	// Get agent bead ID for cross-referencing
+	var agentBeadID string
+	if roleInfo, err := GetRoleWithContext(cwd, townRoot); err == nil {
+		ctx := RoleContext{
+			Role:     roleInfo.Role,
+			Rig:      roleInfo.Rig,
+			Polecat:  roleInfo.Polecat,
+			TownRoot: townRoot,
+			WorkDir:  cwd,
+		}
+		agentBeadID = getAgentBeadID(ctx)
+	}
+
 	// For COMPLETED, we need an issue ID and branch must not be main
 	var mrID string
 	if exitType == ExitCompleted {
@@ -173,6 +186,9 @@ func runDone(cmd *cobra.Command, args []string) error {
 			if worker != "" {
 				description += fmt.Sprintf("\nworker: %s", worker)
 			}
+			if agentBeadID != "" {
+				description += fmt.Sprintf("\nagent_bead: %s", agentBeadID)
+			}
 
 			// Create MR bead (ephemeral wisp - will be cleaned up after merge)
 			mrIssue, err := bd.Create(beads.CreateOptions{
@@ -186,6 +202,13 @@ func runDone(cmd *cobra.Command, args []string) error {
 			}
 			mrID = mrIssue.ID
 
+			// Update agent bead with active_mr reference (for traceability)
+			if agentBeadID != "" {
+				if err := bd.UpdateAgentActiveMR(agentBeadID, mrID); err != nil {
+					style.PrintWarning("could not update agent bead with active_mr: %v", err)
+				}
+			}
+
 			// Success output
 			fmt.Printf("%s Work submitted to merge queue\n", style.Bold.Render("✓"))
 			fmt.Printf("  MR ID: %s\n", style.Bold.Render(mrID))