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>

lib/builtin/settings_ui/src/settings_category_view.dart

@@ -206,7 +206,19 @@ class _Control extends StatelessWidget {
       case SettingsFieldKind.toggle:
         return _ToggleControl(checked: value == true, onChanged: _set);
       case SettingsFieldKind.select:
-        return _SelectControl(field: field, value: value?.toString(), onPick: _set);
+        return _SelectControl(
+          field: field,
+          value: value?.toString(),
+          onPick: (v) {
+            final prefix = field.applyCommandPrefix;
+            if (prefix != null) {
+              // Value selects a command (the subsystem applies + persists).
+              ClideKernel.of(context).commands.execute('$prefix$v');
+            } else {
+              _set(v);
+            }
+          },
+        );
       case SettingsFieldKind.text:
         return _EditControl(field: field, value: value?.toString() ?? '', numeric: false, onCommit: _set);
       case SettingsFieldKind.number: