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.module.ts

@@ -9,10 +9,11 @@ import { AgentConfigsController } from './agent-configs.controller.js';
 import { CoordModule } from '../coord/coord.module.js';
 import { McpClientModule } from '../mcp-client/mcp-client.module.js';
 import { SkillsModule } from '../skills/skills.module.js';
+import { GCModule } from '../gc/gc.module.js';
 
 @Global()
 @Module({
-  imports: [CoordModule, McpClientModule, SkillsModule],
+  imports: [CoordModule, McpClientModule, SkillsModule, GCModule],
   providers: [ProviderService, RoutingService, SkillLoaderService, AgentService],
   controllers: [ProvidersController, SessionsController, AgentConfigsController],
   exports: [AgentService, ProviderService, RoutingService, SkillLoaderService],

apps/gateway/src/agent/agent.service.ts

@@ -26,6 +26,7 @@ import { createWebTools } from './tools/web-tools.js';
 import type { SessionInfoDto } from './session.dto.js';
 import { SystemOverrideService } from '../preferences/system-override.service.js';
 import { PreferencesService } from '../preferences/preferences.service.js';
+import { SessionGCService } from '../gc/session-gc.service.js';
 
 export interface AgentSessionOptions {
   provider?: string;
@@ -101,6 +102,7 @@ export class AgentService implements OnModuleDestroy {
     @Optional()
     @Inject(PreferencesService)
     private readonly preferencesService: PreferencesService | null,
+    @Inject(SessionGCService) private readonly gc: SessionGCService,
   ) {}
 
   /**
@@ -430,6 +432,14 @@ 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> {