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/agent/agent.service.ts

@@ -24,6 +24,7 @@ import { createGitTools } from './tools/git-tools.js';
 import { createShellTools } from './tools/shell-tools.js';
 import { createWebTools } from './tools/web-tools.js';
 import type { SessionInfoDto } from './session.dto.js';
+import { SessionGCService } from '../gc/session-gc.service.js';
 
 export interface AgentSessionOptions {
   provider?: string;
@@ -89,6 +90,7 @@ export class AgentService implements OnModuleDestroy {
     @Inject(CoordService) private readonly coordService: CoordService,
     @Inject(McpClientService) private readonly mcpClientService: McpClientService,
     @Inject(SkillLoaderService) private readonly skillLoaderService: SkillLoaderService,
+    @Inject(SessionGCService) private readonly gc: SessionGCService,
   ) {}
 
   /**
@@ -405,6 +407,14 @@ export class AgentService implements OnModuleDestroy {
     session.listeners.clear();
     session.channels.clear();
     this.sessions.delete(sessionId);
+
+    // Run GC cleanup for this session (fire and forget, errors are logged)
+    this.gc.collect(sessionId).catch((err: unknown) => {
+      this.logger.error(
+        `GC collect failed for session ${sessionId}`,
+        err instanceof Error ? err.stack : String(err),
+      );
+    });
   }
 
   async onModuleDestroy(): Promise<void> {