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",