feat(gateway): SessionGCService three-tier GC + /gc command + cron (P8-014)

Implements three-tier garbage collection for agent sessions:
- SessionGCService.collect() for immediate per-session cleanup on destroySession()
- SessionGCService.sweepOrphans() for daily cron sweep of orphaned Valkey keys
- SessionGCService.fullCollect() for cold-start aggressive cleanup via OnModuleInit
- /gc slash command wired into CommandExecutorService + registered in CommandRegistryService
- SESSION_GC_CRON (daily 4am) added to CronService
- GCModule provides Valkey (ioredis via @mosaic/queue) and is imported by AgentModule, LogModule, CommandsModule, AppModule
- 8 Vitest unit tests covering all three GC tiers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/gateway/src/gc/gc.module.ts

@@ -0,0 +1,31 @@
+import { Module, type OnApplicationShutdown, Inject } from '@nestjs/common';
+import { createQueue, type QueueHandle } from '@mosaic/queue';
+import { SessionGCService } from './session-gc.service.js';
+import { REDIS } from './gc.tokens.js';
+
+const GC_QUEUE_HANDLE = 'GC_QUEUE_HANDLE';
+
+@Module({
+  providers: [
+    {
+      provide: GC_QUEUE_HANDLE,
+      useFactory: (): QueueHandle => {
+        return createQueue();
+      },
+    },
+    {
+      provide: REDIS,
+      useFactory: (handle: QueueHandle) => handle.redis,
+      inject: [GC_QUEUE_HANDLE],
+    },
+    SessionGCService,
+  ],
+  exports: [SessionGCService],
+})
+export class GCModule implements OnApplicationShutdown {
+  constructor(@Inject(GC_QUEUE_HANDLE) private readonly handle: QueueHandle) {}
+
+  async onApplicationShutdown(): Promise<void> {
+    await this.handle.close().catch(() => {});
+  }
+}