feat(ui): scaffold islands shell and child VMs

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

@@ -0,0 +1,11 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui.ViewModels.Islands;
+
+public sealed partial class ListsIslandViewModel : ViewModelBase
+{
+    public event EventHandler? SelectionChanged;
+    [ObservableProperty] private ListNavItemViewModel? _selectedList;
+    partial void OnSelectedListChanged(ListNavItemViewModel? value) =>
+        SelectionChanged?.Invoke(this, EventArgs.Empty);
+}