feat(mcp/external): add UpdateTask for content/tag patching

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -130,6 +130,33 @@ public sealed class ExternalMcpService
         return ToDto(entity);
     }
 
+    [McpServerTool, Description("Update an existing task's title, description, commit type, and/or tags. Pass null to leave a field unchanged. Tags are replaced as a full set when non-null. Refuses if the task is currently Running.")]
+    public async Task<TaskDto> UpdateTask(
+        string taskId,
+        string? title,
+        string? description,
+        string? commitType,
+        IReadOnlyList<string>? tags,
+        CancellationToken cancellationToken)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status == TaskStatus.Running)
+            throw new InvalidOperationException("Cannot update a running task. Cancel it first.");
+
+        if (title is not null) task.Title = title;
+        if (description is not null) task.Description = description;
+        if (commitType is not null) task.CommitType = commitType;
+        await _tasks.UpdateAsync(task, cancellationToken);
+
+        if (tags is not null)
+            await _tasks.SetTagsAsync(taskId, tags, cancellationToken);
+
+        var reload = (await _tasks.GetByIdAsync(taskId, cancellationToken))!;
+        await _broadcaster.TaskUpdated(taskId);
+        return ToDto(reload);
+    }
+
     [McpServerTool, Description("Update a task's status. Only 'Manual' and 'Queued' are permitted — use RunTaskNow or CancelTask for execution control.")]
     public async Task<TaskDto> UpdateTaskStatus(
         string taskId,