feat(ui): dirty Ziel-Verzeichnis beim Oeffnen des Merge-Dialogs melden statt nach dem Fehlschlag

MergeTargetsDto/MergeTargets bekommen TargetDirty (via HasChangesAsync,
includeUntracked: false). Das Merge-Modal zeigt Warnung + Stash-Retry
sofort in InitializeAsync statt erst nach dem dirty_target-Fehlschlag;
der REVIEW-Block in der Detail-Pane zeigt denselben Hinweis neben
Approve. Der dirty_target-Rueckgabepfad bleibt als Absicherung bestehen.

Behebt Befund F6 aus docs/test-session-2026-09-16.md.

src/ClaudeDo.Ui/ViewModels/Islands/MergeSectionViewModel.cs

@@ -28,6 +28,10 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
     [ObservableProperty] private ObservableCollection<string> _mergeTargetBranches = new();
     [ObservableProperty] private string? _selectedMergeTarget;
 
+    // Mirrors MergeTargetsDto.TargetDirty — surfaced next to Approve so a dirty target working
+    // dir is visible before anyone clicks, not just after the merge modal refuses it.
+    [ObservableProperty] private bool _targetDirty;
+
     // No CanExecute to gate here — RefreshMergePreviewAsync is triggered from the Bind path,
     // not a [RelayCommand], so PreviewOp only drives the indicator, never locks a button.
     public OperationStatus PreviewOp { get; } = new();
@@ -141,6 +145,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         MergePreviewText = "";
         MergeIsClean = false;
         MergeIsConflict = false;
+        TargetDirty = false;
         SyncWorktree(null, null, null, null, null);
         SyncTaskContext(null, null, false);
         SyncChildOutcomes(false, 0);