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/package.json

@@ -2,6 +2,7 @@
   "name": "@mosaic/gateway",
   "version": "0.0.0",
   "private": true,
+  "type": "module",
   "main": "dist/main.js",
   "scripts": {
     "build": "tsc",

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

@@ -1,4 +1,4 @@
-import { Controller, Post, Body, Logger, HttpException, HttpStatus } from '@nestjs/common';
+import { Controller, Post, Body, Logger, HttpException, HttpStatus, Inject } from '@nestjs/common';
 import type { AgentSessionEvent } from '@mariozechner/pi-coding-agent';
 import { AgentService } from '../agent/agent.service.js';
 import { v4 as uuid } from 'uuid';
@@ -17,7 +17,7 @@ interface ChatResponse {
 export class ChatController {
   private readonly logger = new Logger(ChatController.name);
 
-  constructor(private readonly agentService: AgentService) {}
+  constructor(@Inject(AgentService) private readonly agentService: AgentService) {}
 
   @Post()
   async chat(@Body() body: ChatRequest): Promise<ChatResponse> {

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');

apps/gateway/tsconfig.json

@@ -5,8 +5,8 @@
     "rootDir": "src",
     "experimentalDecorators": true,
     "emitDecoratorMetadata": true,
-    "module": "CommonJS",
-    "moduleResolution": "Node"
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext"
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "dist"]