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()
     {