pytest CI on GitHub Actions

pytest runs on one core while the runner has eight idle.

Why it happens

pytest is serial unless pytest-xdist is installed and enabled.

What to change

1. Run with -n auto using pytest-xdist
2. Group tests that share fixtures with --dist loadgroup to avoid setup thrash
3. Cache .pytest_cache and the virtualenv
4. Split slow integration tests into a separate job

pytest on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: astral-sh/setup-uv@v5
      - run: uv sync --frozen
      - run: uv run pytest -n auto --dist loadgroup -q

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.