fix(ui): open the merge modal non-modal so a running merge no longer blocks the main window

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs

@@ -5,6 +5,11 @@ namespace ClaudeDo.Ui.Views.Modals;
 
 public partial class MergeModalView : Window
 {
+    /// Open merge windows by task id — the guard against starting the same merge twice.
+    private static readonly Dictionary<string, MergeModalView> OpenWindows = new();
+
+    private readonly TaskCompletionSource _closed = new();
+
     public MergeModalView()
     {
         InitializeComponent();
@@ -17,5 +22,26 @@ public partial class MergeModalView : Window
         };
     }
 
-
+    /// Shows the merge window non-modally so the owner stays usable while the merge (and its verify
+    /// command) runs. Completes when the window closes; a second call for a task whose window is
+    /// already open activates it and waits on that one instead of opening another.
+    public static Task ShowAsync(Window owner, MergeModalViewModel vm)
+    {
+        if (OpenWindows.TryGetValue(vm.TaskId, out var existing))
+        {
+            if (existing.WindowState == WindowState.Minimized)
+                existing.WindowState = WindowState.Normal;
+            existing.Activate();
+            return existing._closed.Task;
+        }
+        var window = new MergeModalView { DataContext = vm };
+        OpenWindows[vm.TaskId] = window;
+        window.Closed += (_, _) =>
+        {
+            OpenWindows.Remove(vm.TaskId);
+            window._closed.TrySetResult();
+        };
+        window.Show(owner);
+        return window._closed.Task;
+    }
 }