feat(mcp): add add_subtask tool to claudedo MCP

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -207,6 +207,44 @@ public sealed class ExternalMcpService
         return ToDto(reload);
     }
 
+    [McpServerTool, Description(
+        "Append a subtask (step) to a task. orderNum defaults to the end. " +
+        "Refuses if the task is currently Running. Subtasks are surfaced to the agent at run time and shown in the task's Steps list.")]
+    public async Task<TaskDto> AddSubtask(
+        string taskId,
+        string title,
+        int? orderNum,
+        CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(title))
+            throw new InvalidOperationException("title is required.");
+
+        await using var ctx = await _dbFactory.CreateDbContextAsync(cancellationToken);
+        var tasks = new TaskRepository(ctx);
+        var subtasks = new SubtaskRepository(ctx);
+
+        var task = await tasks.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status == TaskStatus.Running)
+            throw new InvalidOperationException("Cannot add a subtask to a running task. Cancel it first.");
+
+        var existing = await subtasks.GetByTaskIdAsync(taskId, cancellationToken);
+        var order = orderNum ?? (existing.Count == 0 ? 0 : existing.Max(s => s.OrderNum) + 1);
+
+        await subtasks.AddAsync(new SubtaskEntity
+        {
+            Id = Guid.NewGuid().ToString(),
+            TaskId = taskId,
+            Title = title.Trim(),
+            Completed = false,
+            OrderNum = order,
+            CreatedAt = DateTime.UtcNow,
+        }, cancellationToken);
+
+        await _broadcaster.TaskUpdated(taskId);
+        return ToDto(task);
+    }
+
     [McpServerTool, Description(
         "Update a task's status. Only 'Idle' and 'Queued' are permitted externally — " +
         "use run_task_now or cancel_task for execution control, and review_task to act on a WaitingForReview task. " +