Jest CI on GitHub Actions

Jest transforms every file on every run and uses one worker in CI.

Why it happens

The Jest transform cache is not persisted and --ci defaults are conservative about workers.

What to change

1. Cache the Jest cache directory keyed on the lockfile and config
2. Set --maxWorkers to the core count instead of accepting the default
3. Use --shard across a matrix for suites over ten minutes
4. Switch the transformer to SWC or esbuild — Babel is usually the bottleneck

Jest on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - run: npm ci
      - run: npx jest --ci --maxWorkers=8 --cacheDirectory=/tmp/jest_cache

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.