feat(settings): schema-driven settings engine (T-448)

The core of the settings panel (epic T-444). Subsystems describe a category
as data — a SettingsCategory of carded SettingsSections of SettingsFields
(toggle / select / text / number / opens-external-file), each bound to a
SettingsStore key with help text, a default, and reset-to-default.

Registration is declarative: a new SettingsCategoryContribution carries the
category; the extension manager routes it into a new kernel SettingsRegistry
(exposed on KernelServices), which the panel reads via ClideKernel. Adding a
category is now pure data + a contribution — no widget code.

SettingsCategoryView renders a category into carded sections per ui-design
surface.md: panelHeader card fill, dividerColor border, inputs receding to
panelBackground; select reuses the anchored-overlay menu, text/number commit
on Enter or blur (numeric clamps to bounds). The modal panel now shows the
selected/first registered category, falling back to the empty state.

Tests: registry (sort / dedup / notify), contribution routing on activation,
renderer (render + toggle/select write-through + reset), modal-with-category.

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

lib/kernel/src/facade.dart

@@ -34,6 +34,7 @@ import 'package:clide/kernel/src/recent_files.dart';
 import 'package:clide/kernel/src/scheduler.dart';
 import 'package:clide/kernel/src/secrets.dart';
 import 'package:clide/kernel/src/settings.dart';
+import 'package:clide/kernel/src/settings_registry.dart';
 import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:clide/kernel/src/theme/loader.dart';
 import 'package:clide/kernel/src/toolchain.dart';
@@ -81,10 +82,14 @@ class KernelServices {
     required this.textZoom,
     required this.toast,
     required this.logRing,
+    required this.settingsRegistry,
   });
 
   final Logger log;
   final SettingsStore settings;
+
+  /// Categories registered for the Settings panel (T-444).
+  final SettingsRegistry settingsRegistry;
   final DaemonBus events;
   final MessageBus messages;
 
@@ -155,6 +160,7 @@ class KernelServices {
 
     final settings = SettingsStore(appDir: appDir, onError: (m) => log.warn('settings', m));
     await settings.load();
+    final settingsRegistry = SettingsRegistry();
 
     final i18n = I18n(loader: i18nLoader, log: log, defaultLocale: defaultLocale, initialLocale: initialLocale, availableLocales: availableLocales);
     for (final ns in preloadNamespaces) {
@@ -238,6 +244,7 @@ class KernelServices {
       focus: focus,
       project: project,
       ipc: ipc,
+      settingsRegistry: settingsRegistry,
     );
 
     if (autoStartDaemonClient) {
@@ -248,6 +255,7 @@ class KernelServices {
       log: log,
       logRing: logRing,
       settings: settings,
+      settingsRegistry: settingsRegistry,
       events: events,
       messages: messages,
       filterStates: filterStates,
@@ -287,6 +295,7 @@ class KernelServices {
     await ipc.stop();
     ipc.dispose();
     settings.dispose();
+    settingsRegistry.dispose();
     theme.dispose();
     panels.dispose();
     arrangement.dispose();