feat(settings): Keymap category — preset select (T-451)

The keybindings-ui extension contributes a Keymap SettingsCategory: a preset
select (Default / Vim / VS Code / JetBrains) reading the active preset from
kKeymapPresetSetting. Picking one routes through a new schema affordance —
SettingsField.applyCommandPrefix — running `keymap.preset.<value>`, which calls
KeymapService.setPreset to persist and reload the layered keymap live. The
prefix path keeps the engine generic: settings a subsystem applies via a
command (rather than a bare key write) declare the prefix; the scope tag and
current-value read still use the key.

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

test/builtin/keybindings_ui/preset_commands_test.dart

@@ -3,6 +3,7 @@
 library;
 
 import 'package:clide/builtin/keybindings_ui/keybindings_ui.dart';
+import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -46,4 +47,16 @@ void main() {
     await f.services.commands.execute('keymap.preset.default');
     expect(f.services.settings.get<String>(kKeymapPresetSetting), 'default');
   });
+
+  test('contributes a Keymap settings category routed through the preset commands (T-451)', () {
+    final category = KeybindingsUiExtension().contributions
+        .whereType<SettingsCategoryContribution>()
+        .firstWhere((c) => c.id == 'keymap')
+        .category;
+    expect(category.title, 'Keymap');
+    final field = category.sections.expand((s) => s.fields).firstWhere((f) => f.key == kKeymapPresetSetting);
+    expect(field.kind, SettingsFieldKind.select);
+    expect(field.applyCommandPrefix, 'keymap.preset.');
+    expect(field.options.map((o) => o.value), containsAll(['default', 'vim', 'vscode', 'jetbrains']));
+  });
 }