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.

tests/ClaudeDo.Ui.Tests/ViewModels/MergeModalViewModelTests.cs

@@ -69,6 +69,30 @@ public class MergeModalViewModelTests
         Assert.NotEqual("", vm.CommitMessage);
     }
 
+    [Fact]
+    public async Task Initialize_flags_a_dirty_target_up_front()
+    {
+        var (vm, worker) = Build();
+        worker.Targets = new MergeTargetsDto("main", new[] { "main", "dev" }, "fix(my-list): merge t", TargetDirty: true);
+
+        await vm.InitializeAsync("task-1", "do the thing");
+
+        Assert.True(vm.ShowStashRetry);
+        Assert.False(string.IsNullOrWhiteSpace(vm.WarningMessage));
+    }
+
+    [Fact]
+    public async Task Initialize_does_not_flag_a_clean_target()
+    {
+        var (vm, worker) = Build();
+        worker.Targets = new MergeTargetsDto("main", new[] { "main", "dev" }, "fix(my-list): merge t", TargetDirty: false);
+
+        await vm.InitializeAsync("task-1", "do the thing");
+
+        Assert.False(vm.ShowStashRetry);
+        Assert.Null(vm.WarningMessage);
+    }
+
     [Fact]
     public async Task Submit_shows_a_phase_while_the_merge_is_pending_and_clears_it_after()
     {