chore: enforce ruff format via pre-commit and CI

[eilidhmacnicol]

What

Enforces ruff formatting, which had drifted (11 files were unformatted) and was unenforced anywhere.

• CI: pin ruff==0.15.11 and add a ruff format --check plsdo/ tests/ step to the lint job. Previously CI ran ruff check only — the formatter was never verified, despite CONTRIBUTING.md claiming it was.
• pre-commit: new .pre-commit-config.yaml running ruff-check --fix + ruff-format, pinned to the same v0.15.11 so the hook and CI never disagree. Added pre-commit to the dev extra and documented pre-commit install in CONTRIBUTING.md.
• sty: commit: applies ruff format across plsdo/, tests/, and scripts/ so the new check passes.

Notes

• The ruff version is pinned in three places (pre-commit rev, dev extra, CI) deliberately, to keep them in lockstep. Bump all three together.
• CI's format check is scoped to plsdo/ tests/ (matching the existing ruff check scope); the pre-commit hook covers scripts/ locally.

Verification

• ruff format --check . clean
• ruff check plsdo/ tests/ passes
• pytest tests/ — 208 passed