fix(gateway): load .env from monorepo root via dotenv

Gateway runs from apps/gateway/ via pnpm filter but .env lives at the
monorepo root. Added dotenv to resolve ../../.env first, then local
.env as override.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/gateway/package.json

@@ -20,9 +20,9 @@
     "@mosaic/coord": "workspace:^",
     "@mosaic/db": "workspace:^",
     "@mosaic/discord-plugin": "workspace:^",
-    "@mosaic/telegram-plugin": "workspace:^",
     "@mosaic/log": "workspace:^",
     "@mosaic/memory": "workspace:^",
+    "@mosaic/telegram-plugin": "workspace:^",
     "@mosaic/types": "workspace:^",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
@@ -41,6 +41,7 @@
     "better-auth": "^1.5.5",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.15.1",
+    "dotenv": "^17.3.1",
     "fastify": "^5.0.0",
     "node-cron": "^4.2.1",
     "reflect-metadata": "^0.2.0",