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/command-registry.service.ts

@@ -188,6 +188,14 @@ export class CommandRegistryService implements OnModuleInit {
         execution: 'local',
         available: true,
       },
+      {
+        name: 'gc',
+        description: 'Trigger garbage collection sweep (user-scoped)',
+        aliases: [],
+        scope: 'core',
+        execution: 'socket',
+        available: true,
+      },
     ]);
   }
 }