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/log/cron.service.ts

@@ -7,17 +7,22 @@ import {
 } from '@nestjs/common';
 import cron from 'node-cron';
 import { SummarizationService } from './summarization.service.js';
+import { SessionGCService } from '../gc/session-gc.service.js';
 
 @Injectable()
 export class CronService implements OnModuleInit, OnModuleDestroy {
   private readonly logger = new Logger(CronService.name);
   private readonly tasks: cron.ScheduledTask[] = [];
 
-  constructor(@Inject(SummarizationService) private readonly summarization: SummarizationService) {}
+  constructor(
+    @Inject(SummarizationService) private readonly summarization: SummarizationService,
+    @Inject(SessionGCService) private readonly sessionGC: SessionGCService,
+  ) {}
 
   onModuleInit(): void {
     const summarizationSchedule = process.env['SUMMARIZATION_CRON'] ?? '0 */6 * * *'; // every 6 hours
     const tierManagementSchedule = process.env['TIER_MANAGEMENT_CRON'] ?? '0 3 * * *'; // daily at 3am
+    const gcSchedule = process.env['SESSION_GC_CRON'] ?? '0 4 * * *'; // daily at 4am
 
     this.tasks.push(
       cron.schedule(summarizationSchedule, () => {
@@ -35,8 +40,16 @@ export class CronService implements OnModuleInit, OnModuleDestroy {
       }),
     );
 
+    this.tasks.push(
+      cron.schedule(gcSchedule, () => {
+        this.sessionGC.sweepOrphans().catch((err) => {
+          this.logger.error(`Session GC sweep failed: ${err}`);
+        });
+      }),
+    );
+
     this.logger.log(
-      `Cron scheduled: summarization="${summarizationSchedule}", tier="${tierManagementSchedule}"`,
+      `Cron scheduled: summarization="${summarizationSchedule}", tier="${tierManagementSchedule}", gc="${gcSchedule}"`,
     );
   }
 

apps/gateway/src/log/log.module.ts

@@ -6,9 +6,11 @@ import { LOG_SERVICE } from './log.tokens.js';
 import { LogController } from './log.controller.js';
 import { SummarizationService } from './summarization.service.js';
 import { CronService } from './cron.service.js';
+import { GCModule } from '../gc/gc.module.js';
 
 @Global()
 @Module({
+  imports: [GCModule],
   providers: [
     {
       provide: LOG_SERVICE,