feat(ui): worktree modal with tree view and M/A badges

Adds WorktreeModalView/ViewModel showing git status --porcelain as a
recursive file tree with M/A/D/? status badges. Wires the Worktree
button in AgentStripView to OpenWorktreeCommand on DetailsIslandViewModel.
Adds GetStatusPorcelainAsync to GitService.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,26 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+using ClaudeDo.Ui.ViewModels.Modals;
+
+namespace ClaudeDo.Ui.Views.Modals;
+
+public partial class WorktreeModalView : Window
+{
+    public WorktreeModalView()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnDataContextChanged(EventArgs e)
+    {
+        base.OnDataContextChanged(e);
+        if (DataContext is WorktreeModalViewModel vm)
+            vm.CloseAction = Close;
+    }
+
+    private void OnTitleBarPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
+}