fix(claude-do): merge Make the merge modal non-modal so a running merge no longer

ClaudeDo-Task: b2b4c74fc265435b9f083f56d0e47546

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

@@ -335,4 +335,30 @@ public class DiffViewerViewModelTests
         Assert.Contains("planning task not found", vm.CombinedWarning);
         Assert.Equal("", vm.DisplayedDiff);
     }
+
+    // ── Merge button: the modal is non-modal, the command spans its lifetime ──
+
+    [Fact]
+    public async Task Merge_WhileMergeWindowOpen_DoesNotStartSecondMerge()
+    {
+        var worker = new FakePlanningWorker();
+        var vm = new DiffViewerViewModel(null!, worker, TestSettings());
+        vm.ConfigureWorktree("/wt", "main", "task-1", "Title");
+        var windowClosed = new TaskCompletionSource();
+        var resolved = 0;
+        var shown = 0;
+        vm.ResolveMergeVm = () => { resolved++; return new MergeModalViewModel(worker, new MergeCoordinator()); };
+        vm.ShowMergeModal = _ => { shown++; return windowClosed.Task; };
+
+        var first = vm.MergeCommand.ExecuteAsync(null);
+        while (shown == 0) await Task.Delay(10);
+
+        Assert.False(vm.MergeCommand.CanExecute(null));
+        Assert.Equal(1, resolved);
+        Assert.Equal(1, shown);
+
+        windowClosed.SetResult();
+        await first;
+        Assert.True(vm.MergeCommand.CanExecute(null));
+    }
 }