feat(ui): scaffold islands shell and child VMs

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

@@ -0,0 +1,12 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui.ViewModels.Islands;
+
+public sealed partial class TasksIslandViewModel : ViewModelBase
+{
+    public event EventHandler? SelectionChanged;
+    [ObservableProperty] private TaskRowViewModel? _selectedTask;
+    public void LoadForList(ListNavItemViewModel? list) { /* Phase 5 */ }
+    partial void OnSelectedTaskChanged(TaskRowViewModel? value) =>
+        SelectionChanged?.Invoke(this, EventArgs.Empty);
+}