Disable Vercel deployments for main (production is…

Details

Pushes to main were also triggering a Vercel production build. Add a
vercel.json with git.deploymentEnabled: { "main": false } so Vercel never
creates a deployment for main commits (the live site deploys via GitHub
Pages in ci.yml). Unspecified branches default to true, so Vercel preview
deployments for PRs and other branches are unaffected.
Also document the rule in PREVIEW.md next to the equivalent Netlify
ignore rule.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015WJN8LQbkFiCDVEQaGemqy

Files changed (2)