feat(worker): warn on near-duplicate titles in add_task/batch_add_tasks

add_task and batch_add_tasks now report up to 3 open (non-terminal)
tasks in the same list with a strongly overlapping title, so a
parallel agent can notice and mention a likely duplicate instead of
silently creating one. The task is always created regardless. Uses a
cheap normalized-word overlap heuristic (no embeddings/LLM call),
robust to German umlaut/digraph spelling variants. Breaking change:
AddTask now returns AddTaskResult { task, possibleDuplicates } instead
of a bare TaskRefDto; BatchAddTaskResult gained a PossibleDuplicates
field.

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

@@ -142,6 +142,24 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.Equal(2, inList.Count);
     }
 
+    [Fact]
+    public async Task BatchAddTasks_SimilarTitleInSameList_ReportsPossibleDuplicate()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Idle);
+        var sut = BuildSut();
+
+        var results = await sut.BatchAddTasks(listId, new[]
+        {
+            new BatchAddTaskInput("Settings: MaxTurnsCeiling editierbar machen"),
+        }, cancellationToken: CancellationToken.None);
+
+        var result = Assert.Single(results);
+        Assert.True(result.Ok);
+        Assert.NotNull(result.PossibleDuplicates);
+        Assert.Single(result.PossibleDuplicates!);
+    }
+
     [Fact]
     public async Task BatchGetTasks_MissingId_IsFoundFalseNotError()
     {

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

@@ -1518,7 +1518,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.AddTask(listId, "t", cancellationToken: CancellationToken.None);
 
-        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
         Assert.Null(loaded!.Model);
     }
 
@@ -1530,7 +1530,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.AddTask(listId, "t", model: "HAIKU", cancellationToken: CancellationToken.None);
 
-        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
         Assert.Equal("haiku", loaded!.Model);
     }
 
@@ -1544,6 +1544,80 @@ public sealed class ExternalMcpServiceTests : IDisposable
             () => sut.AddTask(listId, "t", model: "gpt4", cancellationToken: CancellationToken.None));
     }
 
+    // ── AddTask possible-duplicate check ────────────────────────────────────────
+
+    [Fact]
+    public async Task AddTask_SimilarOpenTitleInSameList_IsReportedAsPossibleDuplicate()
+    {
+        // The exact pair that motivated this check (bf4cd901 vs f3718cd8, see task write-up).
+        var listId = await SeedListAsync();
+        var existing = await SeedTaskAsync(
+            listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.WaitingForReview);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        var dup = Assert.Single(result.PossibleDuplicates);
+        Assert.Equal(existing.Id, dup.TaskId);
+        Assert.Equal(existing.Title, dup.Title);
+        Assert.Equal("WaitingForReview", dup.Status);
+    }
+
+    [Fact]
+    public async Task AddTask_StillCreatesTheTask_EvenWhenAPossibleDuplicateIsFound()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        Assert.NotEmpty(result.PossibleDuplicates);
+        Assert.NotNull(await _tasks.GetByIdAsync(result.Task.Id));
+    }
+
+    [Fact]
+    public async Task AddTask_SimilarTitle_ButExistingTaskIsTerminal_IsNotReported()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Done);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        Assert.Empty(result.PossibleDuplicates);
+    }
+
+    [Fact]
+    public async Task AddTask_ClearlyDifferentTitle_IsNotReported()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "Refactor GitService worktree cleanup", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Add dark mode toggle to the settings page", cancellationToken: CancellationToken.None);
+
+        Assert.Empty(result.PossibleDuplicates);
+    }
+
+    [Fact]
+    public async Task AddTask_MoreThanThreeMatches_CapsPossibleDuplicatesAtThree()
+    {
+        var listId = await SeedListAsync();
+        for (var i = 0; i < 5; i++)
+            await SeedTaskAsync(listId, $"Fix flaky queue picker test number {i}", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Fix flaky queue picker test number extra", cancellationToken: CancellationToken.None);
+
+        Assert.Equal(3, result.PossibleDuplicates.Count);
+    }
+
     // ── ContinueTask validation ───────────────────────────────────────────────
 
     [Fact]