feat(worker): add external MCP reset-failed-task tool

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/LifecycleMcpTools.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Lifecycle;
+using ModelContextProtocol.Server;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.External;
+
+[McpServerToolType]
+public sealed class LifecycleMcpTools
+{
+    private readonly TaskRepository _tasks;
+    private readonly TaskResetService _reset;
+
+    public LifecycleMcpTools(TaskRepository tasks, TaskResetService reset)
+    {
+        _tasks = tasks;
+        _reset = reset;
+    }
+
+    [McpServerTool, Description("Reset a failed task: discards its worktree and returns it to Idle so it can be run again. Only Failed tasks are accepted.")]
+    public async Task ResetFailedTask(string taskId, CancellationToken cancellationToken)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status != TaskStatus.Failed)
+            throw new InvalidOperationException($"Task {taskId} is {task.Status}, not Failed. Only failed tasks can be reset via this tool.");
+
+        await _reset.ResetAsync(taskId, cancellationToken);
+    }
+}