Add Vercel deploy workflow for per-PR + v3 preview…

Details

Adds .github/workflows/deploy-vercel.yml. There was no Vercel anywhere in the
repo before — the existing branch previews are Netlify (netlify.toml + the
deploy-preview URL in README/PREVIEW). This wires up Vercel without touching the
live GitHub Pages site (which still deploys only from `main` via ci.yml).
Rather than have Vercel build Jekyll itself (awkward: needs a GitHub token plus a
strict generate-config -> jekyll -> tsc order), the workflow reuses the exact
proven build from preview-v3.yml (Ruby 3.2 + Node 24) and ships the already-built
_site to Vercel as a prebuilt static deployment (Build Output API v3). A push to
`v3` publishes a Vercel production deploy; any PR gets its own preview deploy and
the URL is posted back as a PR comment.
One-time setup (documented in the workflow header): create a Vercel project and
add VERCEL_TOKEN / VERCEL_ORG_ID / VERCEL_PROJECT_ID as GitHub Actions secrets
(plus optional GA_MEASUREMENT_ID). Until VERCEL_TOKEN exists the job no-ops with
a warning instead of failing.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016HyukoK8KXyZw4hY9Wm3zN

Files changed (1)