[CI] Add ruff lint step to CI (+ optional pre-commit config)

[alphacrack]

CI runs pytest but never lints the Python source itself (verified — no ruff/flake8/black anywhere in .github/workflows/ci.yml or pyproject.toml).

Steps:

1. Add ruff to requirements-dev.txt (pinned)
2. Add [tool.ruff] config to pyproject.toml — start permissive: default rules, line-length matching existing style; fix or noqa existing findings in the same PR
3. Add a fast lint job to .github/workflows/ci.yml (single ubuntu leg) and wire it into the gate: ci-ok becomes needs: [test, lint]
4. Optional follow-up PR: .pre-commit-config.yaml with ruff + trailing-whitespace hooks

Good first issue for someone who has set up ruff before. Keep the first PR small — config + green CI, not a style crusade.