Enable Vercel Git deploys for main only.

Disable staging branch deployments in vercel.json and drop the redundant GitHub Actions Vercel deploy step to avoid double production builds.

vercel.json

@@ -2,9 +2,7 @@
   "$schema": "https://openapi.vercel.sh/vercel.json",
   "git": {
     "deploymentEnabled": {
-      "main": false,
       "staging": false
     }
-  },
-  "ignoreCommand": "echo 'Git integration builds disabled; production deploys via GitHub Actions on main only.' && exit 0"
+  }
 }