feat: add audit trail for detach operations (gt-h6eq.8)

Add DetachMoleculeWithAudit() function that logs detach operations
to .beads/audit.log in JSONL format. Each entry captures:
- timestamp
- operation type (detach, burn, squash)
- pinned_bead_id
- detached_molecule
- detached_by (agent identity)
- reason (optional)
- previous_state

Updated callers:
- runMoleculeDetach: logs "detach" operation
- runMoleculeBurn: logs "burn" operation with reason
- runMoleculeSquash: logs "squash" operation with digest ID

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/molecule_lifecycle.go

@@ -201,8 +201,12 @@ func runMoleculeBurn(cmd *cobra.Command, args []string) error {
 
 	moleculeID := attachment.AttachedMolecule
 
-	// Detach the molecule (this "burns" it by removing the attachment)
-	_, err = b.DetachMolecule(handoff.ID)
+	// Detach the molecule with audit logging (this "burns" it by removing the attachment)
+	_, err = b.DetachMoleculeWithAudit(handoff.ID, beads.DetachOptions{
+		Operation: "burn",
+		Agent:     target,
+		Reason:    "molecule burned by agent",
+	})
 	if err != nil {
 		return fmt.Errorf("detaching molecule: %w", err)
 	}
@@ -334,8 +338,12 @@ squashed_at: %s
 			style.Dim.Render("Warning:"), err)
 	}
 
-	// Detach the molecule from the handoff bead
-	_, err = b.DetachMolecule(handoff.ID)
+	// Detach the molecule from the handoff bead with audit logging
+	_, err = b.DetachMoleculeWithAudit(handoff.ID, beads.DetachOptions{
+		Operation: "squash",
+		Agent:     target,
+		Reason:    fmt.Sprintf("molecule squashed to digest %s", digestIssue.ID),
+	})
 	if err != nil {
 		return fmt.Errorf("detaching molecule: %w", err)
 	}