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/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"]