fix: address post-audit review findings across docs and source

docs: add audit plan artifacts and project roadmap

chore(docs): mark Phase-5 tasks complete

chore(config): remove obsolete ruff ignore rules

ci(coverage): enforce 70% coverage threshold in CI

refactor(types): verify type annotations are already tightened

ci(hooks): add pre-commit config with ruff and mypy hooks

ci(deps): consolidate to pyproject.toml, remove requirements files

docs: mark Phase 3 tasks complete

docs: mark Phase 2 tasks complete