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.controller.ts

@@ -0,0 +1,44 @@
+import {
+  Body,
+  Controller,
+  Delete,
+  Get,
+  HttpCode,
+  HttpStatus,
+  Inject,
+  Param,
+  Post,
+  UseGuards,
+} from '@nestjs/common';
+import { PreferencesService } from './preferences.service.js';
+import { AuthGuard } from '../auth/auth.guard.js';
+import { CurrentUser } from '../auth/current-user.decorator.js';
+
+@Controller('api/preferences')
+@UseGuards(AuthGuard)
+export class PreferencesController {
+  constructor(@Inject(PreferencesService) private readonly preferences: PreferencesService) {}
+
+  @Get()
+  async show(@CurrentUser() user: { id: string }): Promise<Record<string, unknown>> {
+    return this.preferences.getEffective(user.id);
+  }
+
+  @Post()
+  @HttpCode(HttpStatus.OK)
+  async set(
+    @CurrentUser() user: { id: string },
+    @Body() body: { key: string; value: unknown },
+  ): Promise<{ success: boolean; message: string }> {
+    return this.preferences.set(user.id, body.key, body.value);
+  }
+
+  @Delete(':key')
+  @HttpCode(HttpStatus.OK)
+  async reset(
+    @CurrentUser() user: { id: string },
+    @Param('key') key: string,
+  ): Promise<{ success: boolean; message: string }> {
+    return this.preferences.reset(user.id, key);
+  }
+}