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/commands/commands.module.ts

@@ -1,8 +1,10 @@
 import { Module } from '@nestjs/common';
 import { CommandRegistryService } from './command-registry.service.js';
 import { CommandExecutorService } from './command-executor.service.js';
+import { GCModule } from '../gc/gc.module.js';
 
 @Module({
+  imports: [GCModule],
   providers: [CommandRegistryService, CommandExecutorService],
   exports: [CommandRegistryService, CommandExecutorService],
 })