Add branch-preview tooling (no impact on the live…

Details

Lets the v3 branch be previewed as a real deployed site without merging to
main (main is the only thing that deploys to GitHub Pages). Verified by a
review workflow + a local build of the exact sequence.
- .github/workflows/preview-v3.yml: builds v3 and uploads _site as a
  downloadable artifact. Has only `contents: read` and never calls
  deploy-pages/upload-pages-artifact, so it cannot change the live site.
  Mirrors ci.yml (Ruby 3.2 + Node 24; generate runtime config -> jekyll ->
  tsc), passes GITHUB_TOKEN so remote_theme/github-metadata don't rate-limit,
  and falls back to a placeholder GA id if the secret is absent.
- netlify.toml: root-served branch preview (publish _site; the post-jekyll tsc
  ordering preserved). Documents the required GA_MEASUREMENT_ID and a
  JEKYLL_GITHUB_TOKEN PAT for remote_theme, plus the Supabase allowed-origins
  caveat.
- .ruby-version (3.2): pins Ruby for Netlify/Cloudflare/local version managers.
- PREVIEW.md: three ways to preview (hosted URL / local serve / Actions
  artifact), including why a subpath preview breaks (root-absolute asset paths)
  and the no-secrets local path via `jekyll serve` (not local.sh, which runs the
  GA4 dashboard that needs secrets).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HShRpQUmDUttxD5F89ziDu

Files changed (4)