feat(mcp/external): add DeleteTask

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

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -219,6 +219,18 @@ public sealed class ExternalMcpService
         return cancelled;
     }
 
+    [McpServerTool, Description("Delete a task. Refuses if the task is currently Running — cancel it first.")]
+    public async Task DeleteTask(string taskId, 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 delete a running task. Cancel it first.");
+
+        await _tasks.DeleteAsync(taskId, cancellationToken);
+        await _broadcaster.TaskUpdated(taskId);
+    }
+
     [McpServerTool, Description("List all known tags. Useful for discovering existing tag names (including 'agent' which marks tasks for auto-execution) before tagging.")]
     public async Task<IReadOnlyList<TagDto>> ListTags(CancellationToken cancellationToken)
     {