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/Modals/MergeModalViewModel.cs

@@ -31,6 +31,10 @@ public sealed partial class MergeModalViewModel : ViewModelBase
     [ObservableProperty] private string? _warningMessage;
     [ObservableProperty] private string? _successMessage;
     [ObservableProperty][NotifyCanExecuteChangedFor(nameof(SubmitCommand))] private bool _hasConflict;
+
+    /// The merge was refused because the target working dir is dirty — offer the stash retry
+    /// instead of dead-ending on the error text.
+    [ObservableProperty] private bool _showStashRetry;
     [ObservableProperty] private IReadOnlyList<string> _conflictFiles = Array.Empty<string>();
 
     public Action? CloseAction { get; set; }
@@ -84,13 +88,21 @@ public sealed partial class MergeModalViewModel : ViewModelBase
         !IsBusy && !HasConflict && !string.IsNullOrWhiteSpace(SelectedBranch);
 
     [RelayCommand(CanExecute = nameof(CanSubmit))]
-    private async Task SubmitAsync()
+    private Task SubmitAsync() => RunMergeAsync(autostash: false);
+
+    /// Retry after "dirty_target": git stashes the target working tree's uncommitted changes,
+    /// merges, and restores them afterwards.
+    [RelayCommand(CanExecute = nameof(CanSubmit))]
+    private Task StashAndMergeAsync() => RunMergeAsync(autostash: true);
+
+    private async Task RunMergeAsync(bool autostash)
     {
         if (string.IsNullOrWhiteSpace(SelectedBranch)) return;
         IsBusy = true;
         ErrorMessage = null;
         WarningMessage = null;
         SuccessMessage = null;
+        ShowStashRetry = false;
         // Subscribed only for the duration of the call: the worker's broadcast reaches every
         // client, and a transient VM left on that event would outlive its window.
         ProgressMessage = Loc.T("vm.merge.progressMerging");
@@ -98,7 +110,7 @@ public sealed partial class MergeModalViewModel : ViewModelBase
         try
         {
             var result = await _worker.MergeTaskAsync(
-                TaskId, SelectedBranch!, RemoveWorktree, CommitMessage);
+                TaskId, SelectedBranch!, RemoveWorktree, CommitMessage, autostash);
 
             switch (result.Status)
             {
@@ -130,6 +142,12 @@ public sealed partial class MergeModalViewModel : ViewModelBase
                     // point of the gate.
                     ErrorMessage = result.ErrorMessage ?? Loc.T("vm.merge.verifyFailed");
                     break;
+                case "dirty_target":
+                    // Nothing landed -- the target working dir has uncommitted changes. Offer the
+                    // stash retry; git restores the changes after the merge.
+                    ErrorMessage = result.ErrorMessage ?? Loc.T("vm.merge.dirtyTarget");
+                    ShowStashRetry = true;
+                    break;
                 case "untracked_collision":
                     // Nothing landed -- the merge was refused before touching the repo. Show the
                     // real message (it names the colliding path and size); the locale string is

src/ClaudeDo.Ui/ViewModels/Modals/WorktreesOverviewModalViewModel.cs

@@ -415,7 +415,8 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     private bool CanMergeAll() => !BatchMergeStatus.IsRunning && SelectedCount > 0 && !string.IsNullOrWhiteSpace(SelectedTarget);
 
     [RelayCommand(CanExecute = nameof(CanMergeAll))]
-    private Task MergeAll() => MergeSelectedAsync(_worker.MergeTaskAsync);
+    private Task MergeAll() => MergeSelectedAsync(
+        (taskId, target, removeWorktree, message) => _worker.MergeTaskAsync(taskId, target, removeWorktree, message));
 
     [RelayCommand]
     private void ResolveConflict(WorktreeOverviewRowViewModel? row)