Replace Pylint and isort with Ruff

[tomsilver]

Replace Pylint, pytest-pylint, and isort with Ruff for linting and import sorting. Configure Ruff's standard error checks, Pyflakes, import sorting, and Pylint rules in pyproject.toml; update the local scripts, GitHub Actions, and README; remove .pylintrc.

Keep Black for code formatting and docformatter for wrapping docstring prose. Ruff's docstring formatting handles embedded code examples. CI now checks docformatter output as well. The formatting script stops on failures and accepts docformatter's successful reformatting exit code.

Ruff's Pylint coverage differs from Pylint's, and this configuration uses Ruff's rule defaults rather than translating every legacy setting. Mypy continues to check types. Fix one membership assertion flagged by Ruff in the repository setup script.

Validation: ./run_ci_checks.sh passes under Python 3.10 in a uv-created .venv after uninstalling Pylint, pytest-pylint, and isort. Black, docformatter, and Ruff checks pass. An isolated smoke check confirms import sorting, docstring wrapping, formatting idempotence, and rejection of an undefined name.