add Zed-style application menu bar: File / View / Help (T-48)

A custom in-window menu bar in the hat (no native menu, D-7), built from
the command registry so it stays in sync and satisfies D-6 parity.

- Menu model + hybrid resolver (menu_model.dart): a curated File/View/Help
  tree where a MenuAutoFill node sweeps in unplaced view.* commands; titles
  + keybindings come from the registry/keymap; unregistered or
  enabledWhen-false items render disabled (greyed), never hidden.
- Widgets: MenuBar row in the hat (chrome tokens), anchored MenuDropdown
  overlay (dropdown tokens), two-column MenuItemRow with inline keybinding.
- Full keyboard: Alt+mnemonic opens (hook in _RootShell._onKey), arrows
  navigate, Enter activates, Esc closes, Left/Right switch menus.
- Commands: file.openFolder / file.newWindow / file.closeWorkspace /
  help.about, registered by MenuBarExtension(services:). File logic lifted
  out of the project switcher into FileActions (one source of truth; the
  switcher now dispatches the commands). Ctrl+O / Ctrl+Shift+N are now real
  keybindings in default.yaml.
- Help → About: version/commit/date/repo from build-info + the bundled
  dependency licenses parsed from assets/licenses.yaml.

Edit/Selection menus are deferred to T-271/T-272 (need focused-surface
command routing).

Tests: resolver + controller + licenses parse (pure); menu-bar widget
(open/close/execute/disabled/Esc/arrow/Enter/Left-Right); FileActions +
Open dialog; app-level Alt+F, non-repo dialog, and closeWorkspace.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/menubar/src/licenses_loader.dart

@@ -0,0 +1,60 @@
+/// Loads the bundled `assets/licenses.yaml` manifest for the About dialog
+/// (T-48 / D-42). The runtime `dependencies:` list is what the About screen
+/// renders; `self:` carries clide's own license.
+library;
+
+import 'package:flutter/services.dart' show rootBundle;
+import 'package:yaml/yaml.dart';
+
+class SelfEntry {
+  const SelfEntry({required this.name, required this.version, required this.license});
+  final String name;
+  final String version;
+  final String license;
+}
+
+class DepEntry {
+  const DepEntry({required this.name, required this.version, required this.license});
+  final String name;
+  final String version;
+  final String license;
+}
+
+class LicensesManifest {
+  const LicensesManifest({required this.self, required this.dependencies});
+  final SelfEntry self;
+  final List<DepEntry> dependencies;
+}
+
+/// Parse the licenses-manifest YAML text. Pure (no asset bundle) so it's
+/// directly unit-testable. Missing fields degrade to '—' rather than throwing.
+LicensesManifest parseLicenses(String yamlText) {
+  final doc = loadYaml(yamlText);
+  final map = doc is YamlMap ? doc : const {};
+  String s(Object? v) => v == null ? '—' : '$v';
+
+  final selfRaw = map['self'];
+  final selfMap = selfRaw is YamlMap ? selfRaw : const {};
+  final self = SelfEntry(name: s(selfMap['name']), version: s(selfMap['version']), license: s(selfMap['license']));
+
+  final deps = <DepEntry>[];
+  final list = map['dependencies'];
+  if (list is YamlList) {
+    for (final e in list) {
+      if (e is YamlMap) {
+        deps.add(DepEntry(name: s(e['name']), version: s(e['version']), license: s(e['license'])));
+      }
+    }
+  }
+  return LicensesManifest(self: self, dependencies: deps);
+}
+
+LicensesManifest? _cache;
+
+/// Load + parse the bundled manifest, cached after the first read.
+Future<LicensesManifest> loadLicenses() async {
+  final cached = _cache;
+  if (cached != null) return cached;
+  final raw = await rootBundle.loadString('assets/licenses.yaml');
+  return _cache = parseLicenses(raw);
+}