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/system-override.service.ts

@@ -0,0 +1,33 @@
+import { Injectable, Logger } from '@nestjs/common';
+import { createQueue, type QueueHandle } from '@mosaic/queue';
+
+const SESSION_SYSTEM_KEY = (sessionId: string) => `mosaic:session:${sessionId}:system`;
+const TTL_SECONDS = 5 * 60; // 5 minutes, renewed on each turn
+
+@Injectable()
+export class SystemOverrideService {
+  private readonly logger = new Logger(SystemOverrideService.name);
+  private readonly handle: QueueHandle;
+
+  constructor() {
+    this.handle = createQueue();
+  }
+
+  async set(sessionId: string, override: string): Promise<void> {
+    await this.handle.redis.setex(SESSION_SYSTEM_KEY(sessionId), TTL_SECONDS, override);
+    this.logger.debug(`Set system override for session ${sessionId} (TTL=${TTL_SECONDS}s)`);
+  }
+
+  async get(sessionId: string): Promise<string | null> {
+    return this.handle.redis.get(SESSION_SYSTEM_KEY(sessionId));
+  }
+
+  async renew(sessionId: string): Promise<void> {
+    await this.handle.redis.expire(SESSION_SYSTEM_KEY(sessionId), TTL_SECONDS);
+  }
+
+  async clear(sessionId: string): Promise<void> {
+    await this.handle.redis.del(SESSION_SYSTEM_KEY(sessionId));
+    this.logger.debug(`Cleared system override for session ${sessionId}`);
+  }
+}