ci: add PR check workflow for lint, typecheck, and build

[samholmes]

Summary

• Adds a new check.yml GitHub Actions workflow that runs on every pull request targeting main
• Runs three checks sequentially: lint (bun run lint), type check (bunx tsc --noEmit), and build (bun run build)
• Uses concurrency groups per branch to cancel outdated runs when new commits are pushed
• Catches build failures (like the recent Suspense boundary issue) before they hit main

[tomatrow]

Looks goo to me