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/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,