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-executor.service.ts

@@ -3,6 +3,7 @@ import type { SlashCommandPayload, SlashCommandResultPayload } from '@mosaic/typ
 import { AgentService } from '../agent/agent.service.js';
 import { CommandRegistryService } from './command-registry.service.js';
 import { SystemOverrideService } from '../preferences/system-override.service.js';
+import { SessionGCService } from '../gc/session-gc.service.js';
 
 @Injectable()
 export class CommandExecutorService {
@@ -12,9 +13,10 @@ export class CommandExecutorService {
     @Inject(CommandRegistryService) private readonly registry: CommandRegistryService,
     @Inject(AgentService) private readonly agentService: AgentService,
     @Inject(SystemOverrideService) private readonly systemOverride: SystemOverrideService,
+    @Inject(SessionGCService) private readonly sessionGC: SessionGCService,
   ) {}
 
-  async execute(payload: SlashCommandPayload, _userId: string): Promise<SlashCommandResultPayload> {
+  async execute(payload: SlashCommandPayload, userId: string): Promise<SlashCommandResultPayload> {
     const { command, args, conversationId } = payload;
 
     const def = this.registry.getManifest().commands.find((c) => c.name === command);
@@ -63,6 +65,16 @@ export class CommandExecutorService {
             success: true,
             message: 'Retry last message requested.',
           };
+        case 'gc': {
+          // User-scoped sweep for non-admin; system-wide for admin
+          const result = await this.sessionGC.sweepOrphans(userId);
+          return {
+            command: 'gc',
+            success: true,
+            message: `GC sweep complete: ${result.orphanedSessions} orphaned sessions cleaned in ${result.duration}ms.`,
+            conversationId,
+          };
+        }
         default:
           return {
             command,