Add prek-based pre-commit lint hook

[maxachis]

Summary

• add a prek pre-commit configuration that runs flake8 with existing plugin set
• move flake8 ignore settings into shared .flake8 config and point reviewdog workflow at flake8 dot
• add prek and flake8 tooling to the dev dependency group and update contributor/testing docs

Validation

• uv lock
• uv sync --group dev
• uv run prek install
• uv run prek run --all-files (fails due to pre-existing repository lint violations; hook wiring/config is functional)

[josh-chamberlain]

we like pre-commit linting