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/app.module.ts

@@ -18,6 +18,7 @@ import { PluginModule } from './plugin/plugin.module.js';
 import { McpModule } from './mcp/mcp.module.js';
 import { AdminModule } from './admin/admin.module.js';
 import { CommandsModule } from './commands/commands.module.js';
+import { PreferencesModule } from './preferences/preferences.module.js';
 import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
 
 @Module({
@@ -39,6 +40,7 @@ import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
     PluginModule,
     McpModule,
     AdminModule,
+    PreferencesModule,
     CommandsModule,
   ],
   controllers: [HealthController],