Add a pre-commit config #119

There is no .pre-commit-config.yaml. Contributors find out about lint and format failures from CI, minutes after pushing, when ruff would have told them in two seconds locally.

Add one running ruff check --fix and ruff format over sdk/, matching the versions already pinned in sdk/pyproject.toml so local and CI agree. Add a line to CONTRIBUTING under Local checks telling people to run pre-commit install.

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