feat(gateway): PreferencesService + /preferences REST + /system Valkey override (P8-011)

- PreferencesService: platform defaults, user overrides, IMMUTABLE_KEYS enforcement
- PreferencesController: GET /api/preferences, POST /api/preferences, DELETE /api/preferences/:key
- PreferencesModule: global module exporting PreferencesService and SystemOverrideService
- SystemOverrideService: Valkey-backed session-scoped system prompt override with 5-min TTL + renew
- CommandRegistryService: register /system command (socket execution)
- CommandExecutorService: handle /system command via SystemOverrideService
- AgentService: inject system override before each prompt turn, renew TTL; store userId in session
- ChatGateway: pass userId when creating agent sessions
- PreferencesService unit tests: 11 tests covering defaults, overrides, enforcement wins, immutable key errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/preferences/preferences.module.ts

@@ -0,0 +1,12 @@
+import { Global, Module } from '@nestjs/common';
+import { PreferencesService } from './preferences.service.js';
+import { PreferencesController } from './preferences.controller.js';
+import { SystemOverrideService } from './system-override.service.js';
+
+@Global()
+@Module({
+  controllers: [PreferencesController],
+  providers: [PreferencesService, SystemOverrideService],
+  exports: [PreferencesService, SystemOverrideService],
+})
+export class PreferencesModule {}