Make Vercel deploys manual-only; add Vercel badge…

Details

The deploy-vercel.yml workflow (shipped in #59) deployed to Vercel on
every PR and every v3 push, generating bot-comment email noise. PR
previews are already covered by Netlify's free deploy previews, so drop
the automatic triggers: the workflow now runs only via workflow_dispatch,
with a `production` boolean input to update kricheljgithubio.vercel.app
on purpose. Remove the now-dead PR comment step, its pull-requests:write
permission, and the fork guard.
Also add the Vercel deploy badge (deploy-badge.vercel.app, verified
rendering) to the README next to the Netlify badge, and update
PREVIEW.md's Vercel section to describe the on-demand flow.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015WJN8LQbkFiCDVEQaGemqy

Files changed (3)