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)
     {

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -225,4 +225,40 @@ public sealed class ExternalMcpServiceTests : IDisposable
         await Assert.ThrowsAsync<InvalidOperationException>(() =>
             sut.UpdateTask("does-not-exist", "x", null, null, null, CancellationToken.None));
     }
+
+    [Fact]
+    public async Task DeleteTask_RemovesTaskAndTagJoins()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId);
+        await _tasks.SetTagsAsync(task.Id, new[] { "agent" });
+        var queue = CreateQueue();
+        var sut = BuildSut(queue);
+
+        await sut.DeleteTask(task.Id, CancellationToken.None);
+
+        Assert.Null(await _tasks.GetByIdAsync(task.Id));
+    }
+
+    [Fact]
+    public async Task DeleteTask_OnRunning_Throws()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId, status: TaskStatus.Running);
+        var queue = CreateQueue();
+        var sut = BuildSut(queue);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            sut.DeleteTask(task.Id, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task DeleteTask_NotFound_Throws()
+    {
+        var queue = CreateQueue();
+        var sut = BuildSut(queue);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            sut.DeleteTask("does-not-exist", CancellationToken.None));
+    }
 }