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/Program.cs

@@ -35,7 +35,6 @@ sealed class Program
             SetCurrentProcessExplicitAppUserModelID("ClaudeDo.App");
 
         var services = BuildServices();
-        App.Services = services;
 
         using (var scope = services.CreateScope())
         {
@@ -45,7 +44,7 @@ sealed class Program
 
         try
         {
-            BuildAvaloniaApp()
+            ConfigureAppBuilder(AppBuilder.Configure(() => new App(services)))
                 .StartWithClassicDesktopLifetime(args);
         }
         finally
@@ -58,8 +57,12 @@ sealed class Program
         }
     }
 
+    // Parameterless entry point required by the XAML previewer / designer.
     public static AppBuilder BuildAvaloniaApp()
-        => AppBuilder.Configure<App>()
+        => ConfigureAppBuilder(AppBuilder.Configure<App>());
+
+    private static AppBuilder ConfigureAppBuilder(AppBuilder builder)
+        => builder
             .UsePlatformDetect()
 #if DEBUG
             .WithDeveloperTools()