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/kernel/src/settings_schema.dart

@@ -52,6 +52,7 @@ class SettingsField {
     this.min,
     this.max,
     this.fileCommand,
+    this.applyCommandPrefix,
   });
 
   final String key;
@@ -73,6 +74,13 @@ class SettingsField {
 
   /// For [SettingsFieldKind.file]: the command id the row's button invokes.
   final String? fileCommand;
+
+  /// For [SettingsFieldKind.select]: when set, picking option `<value>` runs
+  /// the command `<applyCommandPrefix><value>` instead of writing [key]
+  /// directly — for settings a subsystem applies via a command (and only then
+  /// persists). The current value is still read from [key], so the scope tag
+  /// and selection still work. Example: `'keymap.preset.'` → `keymap.preset.vim`.
+  final String? applyCommandPrefix;
 }
 
 /// A carded group of fields (surface.md "sectioned cards"). [label] is the