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/app.module.ts

@@ -19,6 +19,7 @@ import { McpModule } from './mcp/mcp.module.js';
 import { AdminModule } from './admin/admin.module.js';
 import { CommandsModule } from './commands/commands.module.js';
 import { PreferencesModule } from './preferences/preferences.module.js';
+import { GCModule } from './gc/gc.module.js';
 import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
 
 @Module({
@@ -42,6 +43,7 @@ import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
     AdminModule,
     PreferencesModule,
     CommandsModule,
+    GCModule,
   ],
   controllers: [HealthController],
   providers: [