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/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -1275,7 +1275,21 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         {
             var hasChildren = Subtasks.Count > 0 || ChildOutcomes.Count > 0;
             var result = await _worker.ApproveReviewAsync(taskId, Merge.SelectedMergeTarget ?? "");
-            if (!hasChildren && result?.Status == "conflict")
+
+            // Nothing landed -- the target working dir has uncommitted changes (often another
+            // session's WIP in the shared main checkout). Offer the one-shot stash retry; git
+            // restores the changes itself once the merge is through.
+            if (result?.Status == "dirty_target" && ConfirmAsync != null
+                && await ConfirmAsync(Loc.T("vm.detailsIsland.stashAndMergeConfirm")))
+            {
+                ApproveOp.Report(Loc.T("ops.merge.merging"));
+                result = await _worker.ApproveReviewAsync(
+                    taskId, Merge.SelectedMergeTarget ?? "", autostash: true);
+            }
+
+            if (result?.Status == "dirty_target" && ShowErrorAsync != null)
+                await ShowErrorAsync(result.ErrorMessage ?? Loc.T("vm.detailsIsland.dirtyTarget"));
+            else if (!hasChildren && result?.Status == "conflict")
                 await _merge.ResolveConflictAsync(taskId, Merge.SelectedMergeTarget ?? "");
             // The merge itself already landed; the verify command failed, so the task stayed
             // out of Done. Surface that instead of silently looking like nothing happened.