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/BatchMcpTools.cs

@@ -93,8 +93,8 @@ public sealed class BatchMcpTools
     }
 
     [McpServerTool, Description(
-        "Set the status of many tasks at once. status is 'Idle' (reset to editable) or " +
-        "'Queued' (enqueue for execution) only — same rule as update_task_status. " +
+        "Set the status of many tasks at once. status is 'Idle', 'Queued', 'Cancelled' or 'Done' only — " +
+        "same rule as update_task_status ('Done' is refused per-item for a task with an active worktree). " +
         "Returns one result per id: { taskId, ok, error }. Max 100 ids.")]
     public async Task<IReadOnlyList<BatchTaskResult>> BatchUpdateTaskStatus(
         string[] taskIds, string status, CancellationToken cancellationToken)