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/DetailsIslandViewModel.cs

@@ -726,6 +726,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
                     Merge.MergeTargetBranches.Clear();
                     foreach (var b in targets.LocalBranches) Merge.MergeTargetBranches.Add(b);
                     Merge.SelectedMergeTarget = targets.DefaultBranch;
+                    Merge.TargetDirty = targets.TargetDirty;
                 }
             }
             await Merge.RefreshMergePreviewAsync();
@@ -776,6 +777,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
                         foreach (var b in targets.LocalBranches)
                             Merge.MergeTargetBranches.Add(b);
                         Merge.SelectedMergeTarget = targets.DefaultBranch;
+                        Merge.TargetDirty = targets.TargetDirty;
                     }
                 }
             }
@@ -822,6 +824,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
                         foreach (var b in targets.LocalBranches)
                             Merge.MergeTargetBranches.Add(b);
                         Merge.SelectedMergeTarget = targets.DefaultBranch;
+                        Merge.TargetDirty = targets.TargetDirty;
                     }
                 }
             }

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);