vercel.json — point GitHub-triggered builds at site/

The previous push deployed an empty production build (404) because
GitHub-triggered builds use the repo root, but index.html lives in
site/. Manual `vercel --prod` runs from inside site/ worked, but the
auto-deploy needs an explicit outputDirectory.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vercel.json

@@ -0,0 +1,6 @@
+{
+  "outputDirectory": "site",
+  "framework": null,
+  "buildCommand": null,
+  "installCommand": null
+}