fix: only allow deploy hook triggered builds via ignoreCommand

Skip auto-deploy on push while allowing deploy hook triggered builds
by checking VERCEL_DEPLOY_HOOK_NAME environment variable.

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

Author: tianzhou

vercel.json

@@ -1,2 +1,3 @@
 {
+  "ignoreCommand": "if [ -n \"$VERCEL_DEPLOY_HOOK_NAME\" ]; then exit 1; else exit 0; fi"
 }