test(files): lock .worktrees/ into the ignore roster + init scaffold

The 23dd7fd1 behavior shipped without a test delta; the builtin-roster
lock test and the scaffold assertion now cover it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/files/ignore_test.dart

@@ -61,7 +61,7 @@ void main() {
 
     test('built-in set hides clide-owned dirs', () {
       final s = IgnoreSet.builtin();
-      for (final d in const ['.git', '.pql', '.clide', '.dart_tool', 'build', 'node_modules']) {
+      for (final d in const ['.git', '.pql', '.clide', '.dart_tool', '.worktrees', 'build', 'node_modules']) {
         expect(s.isIgnored(d, isDirectory: true), isTrue, reason: d);
       }
       expect(s.isIgnored('lib', isDirectory: true), isFalse);