fix: switch gateway to ESM + explicit @Inject for tsx compatibility

Pi SDK is ESM-only. tsx (esbuild) doesn't emit decorator metadata,
so NestJS constructor injection fails without explicit @Inject().

- Set "type": "module" in gateway package.json
- Switch tsconfig to NodeNext module resolution
- Add @Inject(AgentService) to ChatController and ChatGateway

Tested end-to-end: REST /api/chat → Pi SDK → Anthropic → response OK.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/src/chat/chat.gateway.ts

@@ -1,4 +1,4 @@
-import { Logger } from '@nestjs/common';
+import { Inject, Logger } from '@nestjs/common';
 import {
   WebSocketGateway,
   WebSocketServer,
@@ -33,7 +33,7 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     { conversationId: string; cleanup: () => void }
   >();
 
-  constructor(private readonly agentService: AgentService) {}
+  constructor(@Inject(AgentService) private readonly agentService: AgentService) {}
 
   afterInit(): void {
     this.logger.log('Chat WebSocket gateway initialized');