Add a pre-commit hook for ruff check + format

## Task

CI runs `ruff check` and `ruff format --check`, but there is no `.pre-commit-config.yaml`. Easy way to fail locally before push.

## Scope

- Add `.pre-commit-config.yaml` with ruff (check + format)
- Document `pre-commit install` in CONTRIBUTING.md
- Pin a current ruff rev compatible with `pyproject.toml`

## Acceptance criteria

- [ ] Hook is optional (do not make it mandatory in CI beyond existing ruff jobs)
- [ ] CONTRIBUTING mentions it
- [ ] `ruff check src tests` still matches CI

Start by comparing the existing CI ruff commands with pyproject.toml, then inspect CONTRIBUTING.md. Add the optional .pre-commit-config.yaml with a pinned compatible ruff revision and document pre-commit install; done means the documentation is present and ruff check src tests still matches CI.

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