feat(merge): Stash-Option fuer uncommittete Aenderungen im Ziel-Tree

Ein dirty Ziel-Arbeitsverzeichnis gibt jetzt den eigenen Status
dirty_target zurueck statt blocked. Die UI bietet daraufhin einen
Retry an, der per git merge --autostash stasht, merged und die
Aenderungen danach wiederherstellt - opt-in pro Merge:

- Approve & Merge: Bestaetigungsdialog
- Merge-Modal: Button "Aenderungen stashen und mergen"
- MCP (merge_task/review_task) wirft weiterhin, kein autostash dort

tests/ClaudeDo.Ui.Tests/ViewModels/MergeModalViewModelTests.cs

@@ -21,17 +21,22 @@ public class MergeModalViewModelTests
         public TaskCompletionSource<MergeResultDto> MergeGate { get; } = new();
         public bool BlockMerge { get; set; }
         public string? CapturedCommitMessage { get; private set; }
+        public bool CapturedAutostash { get; private set; }
+
+        /// Result for a call with autostash=false; the autostash retry always merges.
+        public MergeResultDto CleanResult { get; set; } = new("merged", new List<string>(), null);
 
         public override Task<MergeTargetsDto?> GetMergeTargetsAsync(string taskId) =>
             Task.FromResult(Targets);
 
         public override Task<MergeResultDto> MergeTaskAsync(
-            string taskId, string targetBranch, bool removeWorktree, string commitMessage)
+            string taskId, string targetBranch, bool removeWorktree, string commitMessage, bool autostash = false)
         {
             CapturedCommitMessage = commitMessage;
+            CapturedAutostash = autostash;
             return BlockMerge
                 ? MergeGate.Task
-                : Task.FromResult(new MergeResultDto("merged", new List<string>(), null));
+                : Task.FromResult(autostash ? new MergeResultDto("merged", new List<string>(), null) : CleanResult);
         }
     }
 
@@ -90,6 +95,26 @@ public class MergeModalViewModelTests
         Assert.True(vm.Merged);
     }
 
+    [Fact]
+    public async Task Dirty_target_offers_the_stash_retry_and_merges_with_autostash()
+    {
+        var (vm, worker) = Build();
+        await vm.InitializeAsync("task-1", "do the thing");
+        worker.CleanResult = new MergeResultDto("dirty_target", new List<string>(), "target working tree has uncommitted changes");
+
+        await vm.SubmitCommand.ExecuteAsync(null);
+
+        Assert.True(vm.ShowStashRetry);
+        Assert.False(vm.Merged);
+        Assert.False(worker.CapturedAutostash);
+
+        await vm.StashAndMergeCommand.ExecuteAsync(null);
+
+        Assert.True(worker.CapturedAutostash);
+        Assert.True(vm.Merged);
+        Assert.False(vm.ShowStashRetry);
+    }
+
     [Fact]
     public async Task Progress_for_another_task_is_ignored()
     {