chore(ci): add PR workflow for build, typecheck, format & test

[mihar-22]

Summary

Adds a GitHub Actions workflow that runs on every pull request and push to main. Validates the full pipeline:

1. Format check — oxfmt --check across both packages
2. Typecheck — tsgo --noEmit against tsconfig.json in each package
3. Build — tsdown JS bundles + tsgo-generated bundled .d.ts declarations via turbo
4. Test — vitest --run

Workflow design

• Triggers: pull_request to main + push to main (so the main branch is also continuously validated)
• Single job, sequential steps: each step fails fast on errors. Splitting into parallel jobs would re-install deps + re-build for each, which is wasteful for such a fast pipeline (~25s end-to-end based on local runs)
• Concurrency: cancels in-progress runs on the same PR when new commits land
• Node 22 (required by tsdown >= 22.18) — was previously 18 in the weekly workflow
• pnpm 8.7.0 to match packageManager field
• pnpm install --frozen-lockfile to catch lockfile drift

Additional changes

• Added format:check script to root (turbo run format:check --parallel) and both packages (oxfmt --check src)
• Added format:check turbo task with the same inputs as format

Why not put format/typecheck in separate jobs?

The full pipeline runs in ~25 seconds locally, which is fast enough that the overhead of duplicate pnpm install (~30-60s) across parallel jobs would actually be slower. A single job that fails fast is the right tradeoff here.

Local verification

Step	Time
pnpm format:check	~1.2s
pnpm typecheck	~1s
pnpm build	~5s
pnpm test	~2s

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