feat(mcp): allow Done via update_task_status with worktree guard

External tasks finished outside a ClaudeDo run had no way to close out
their tracking task; update_task_status now permits Done alongside
Idle/Queued/Cancelled, refusing it when the task has an active
worktree so review_task stays the only path that merges.

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -264,10 +264,12 @@ public sealed class ExternalMcpService
     }
 
     [McpServerTool, Description(
-        "Update a task's status. Only 'Idle', 'Queued' and 'Cancelled' are permitted externally — " +
+        "Update a task's status. Only 'Idle', 'Queued', 'Cancelled' and 'Done' are permitted externally — " +
         "use run_task_now for execution control, and review_task to act on a WaitingForReview task. " +
         "Settable: Idle (reset to editable), Queued (enqueue for execution), " +
-        "Cancelled (retire the task without deleting it; it can be reset to Idle later). " +
+        "Cancelled (retire the task without deleting it; it can be reset to Idle later), " +
+        "Done (mark complete; refused if the task has an active worktree — use review_task to approve " +
+        "and merge that worktree instead). " +
         "Full lifecycle: Idle → Queued → Running → WaitingForReview → Done | Failed | Cancelled.")]
     public async Task<TaskDto> UpdateTaskStatus(
         string taskId,
@@ -300,6 +302,20 @@ public sealed class ExternalMcpService
                     throw new InvalidOperationException(cancelResult.Reason ?? "Cannot cancel task.");
                 break;
 
+            case TaskStatus.Done:
+                await using (var ctx = await _dbFactory.CreateDbContextAsync(cancellationToken))
+                {
+                    var wt = await new WorktreeRepository(ctx).GetByTaskIdAsync(taskId, cancellationToken);
+                    if (wt is not null && wt.State == WorktreeState.Active)
+                        throw new InvalidOperationException(
+                            "Task has an active worktree — use review_task to approve and merge instead.");
+                }
+
+                var doneResult = await _state.ForceSetStatusAsync(taskId, TaskStatus.Done, cancellationToken);
+                if (!doneResult.Ok)
+                    throw new InvalidOperationException(doneResult.Reason ?? "Cannot set task to Done.");
+                break;
+
             default:
                 throw new InvalidOperationException(
                     $"Status '{target}' is not settable externally. Use run_task_now or review_task.");