refactor: address code smells (run-dir helper, App DI injection)

- TaskRunner: extract worktree-vs-sandbox selection into
  PrepareRunDirectoryAsync so RunAsync reads linearly (a small helper, not
  a Strategy pattern — overkill for a two-way branch).
- App: drop the public static ServiceProvider locator; inject the provider
  via constructor through AppBuilder.Configure(() => new App(services)).
  Parameterless ctor + BuildAvaloniaApp() retained for the XAML designer.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.App/App.axaml.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
@@ -10,7 +11,12 @@ namespace ClaudeDo.App;
 
 public partial class App : Application
 {
-    public static ServiceProvider Services { get; set; } = null!;
+    private readonly IServiceProvider? _services;
+
+    // Parameterless ctor is required by the XAML previewer / designer.
+    public App() { }
+
+    public App(IServiceProvider services) => _services = services;
 
     public override void Initialize()
     {
@@ -21,16 +27,19 @@ public partial class App : Application
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
+            var services = _services
+                ?? throw new InvalidOperationException("App was constructed without a service provider.");
+
             FocusClearing.Install();
 
             desktop.MainWindow = new MainWindow
             {
-                DataContext = Services.GetRequiredService<IslandsShellViewModel>(),
+                DataContext = services.GetRequiredService<IslandsShellViewModel>(),
             };
 
             // Kick off the SignalR retry loop — reconnects indefinitely if the worker
             // is not up yet, or goes down and comes back.
-            _ = Services.GetRequiredService<WorkerClient>().StartAsync();
+            _ = services.GetRequiredService<WorkerClient>().StartAsync();
         }
 
         base.OnFrameworkInitializationCompleted();