feat(done): implement self-nuke for polecats (self-cleaning model)

When a polecat runs `gt done` with COMPLETED status, it now nukes its own
worktree before exiting. This is the self-cleaning model - polecats clean
up after themselves, reducing Witness/Deacon cleanup burden.

The self-nuke is:
- Only attempted for polecats (not Mayor/Witness/Deacon/Refinery)
- Only on COMPLETED status (not ESCALATED/DEFERRED)
- Non-fatal: if it fails, Witness will handle cleanup

Closes: gt-fqcst

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

internal/cmd/done.go

@@ -343,10 +343,27 @@ func runDone(cmd *cobra.Command, args []string) error {
 	// Update agent bead state (ZFC: self-report completion)
 	updateAgentStateOnDone(cwd, townRoot, exitType, issueID)
 
+	// Self-cleaning: Nuke our own sandbox before exiting (if we're a polecat)
+	// This is the self-cleaning model - polecats clean up after themselves
+	selfNukeAttempted := false
+	if exitType == ExitCompleted {
+		if roleInfo, err := GetRoleWithContext(cwd, townRoot); err == nil && roleInfo.Role == RolePolecat {
+			selfNukeAttempted = true
+			if err := selfNukePolecat(roleInfo, townRoot); err != nil {
+				// Non-fatal: Witness will clean up if we fail
+				style.PrintWarning("self-nuke failed: %v (Witness will clean up)", err)
+			} else {
+				fmt.Printf("%s Sandbox nuked\n", style.Bold.Render("✓"))
+			}
+		}
+	}
+
 	// Always exit session - polecats don't stay alive after completion
 	fmt.Println()
 	fmt.Printf("%s Session exiting (done means gone)\n", style.Bold.Render("→"))
-	fmt.Printf("  Witness will handle worktree cleanup.\n")
+	if !selfNukeAttempted {
+		fmt.Printf("  Witness will handle worktree cleanup.\n")
+	}
 	fmt.Printf("  Goodbye!\n")
 	os.Exit(0)
 
@@ -474,3 +491,29 @@ func parseCleanupStatus(s string) polecat.CleanupStatus {
 		return polecat.CleanupUnknown
 	}
 }
+
+// selfNukePolecat deletes this polecat's worktree (self-cleaning model).
+// Called by polecats when they complete work via `gt done`.
+// This is safe because:
+// 1. Work has been pushed to origin (MR is in queue)
+// 2. We're about to exit anyway
+// 3. Unix allows deleting directories while processes run in them
+func selfNukePolecat(roleInfo RoleInfo, _ string) error {
+	if roleInfo.Role != RolePolecat || roleInfo.Polecat == "" || roleInfo.Rig == "" {
+		return fmt.Errorf("not a polecat: role=%s, polecat=%s, rig=%s", roleInfo.Role, roleInfo.Polecat, roleInfo.Rig)
+	}
+
+	// Get polecat manager using existing helper
+	mgr, _, err := getPolecatManager(roleInfo.Rig)
+	if err != nil {
+		return fmt.Errorf("getting polecat manager: %w", err)
+	}
+
+	// Use nuclear=true since we know we just pushed our work
+	// The branch is pushed, MR is created, we're clean
+	if err := mgr.RemoveWithOptions(roleInfo.Polecat, true, true); err != nil {
+		return fmt.Errorf("removing worktree: %w", err)
+	}
+
+	return nil
+}