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

src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs

@@ -110,9 +110,11 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// and clears BlockedByTaskId in the same update) — call once per row (parent and each
     /// queued child) rather than cascading server-side.</summary>
     Task DequeueTaskAsync(string taskId);
-    Task<MergeResultDto?> ApproveReviewAsync(string taskId, string targetBranch);
+    /// <param name="autostash">Retry flag for status "dirty_target": stash the target working
+    /// tree's uncommitted changes before merging and restore them afterwards (git --autostash).</param>
+    Task<MergeResultDto?> ApproveReviewAsync(string taskId, string targetBranch, bool autostash = false);
     Task<MergePreviewDto?> PreviewMergeAsync(string taskId, string targetBranch);
-    Task<MergeResultDto> MergeTaskAsync(string taskId, string targetBranch, bool removeWorktree, string commitMessage);
+    Task<MergeResultDto> MergeTaskAsync(string taskId, string targetBranch, bool removeWorktree, string commitMessage, bool autostash = false);
     Task RejectReviewToQueueAsync(string taskId, string feedback);
     Task RejectReviewToIdleAsync(string taskId);
     Task CancelReviewAsync(string taskId);