Add pre-merge PR validation workflow

Details

ci.yml only runs on push to main (after merge), so PRs were never validated
before merging. This workflow runs the fast, deterministic checks on every
pull_request: TypeScript build, Jekyll build, SCSS compile, the uv lockfile
smoke-import, and (same-repo PRs only) the Supabase REST smoke test. Slow/flaky
external-link crawling and the live GA4 browser test stay on the post-merge run.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9nJYCSwtVUrxubiU4VAMN

Files changed (1)