chore: adopt ruff for linting, formatting and import sorting

[refeed]

The project currently uses black (pyproject.toml's [tool.black], black in the Pipfile) and
has no lint or import-sorting step. Ruff covers all three in one
fast tool: ruff check for linting (including isort-style import sorting via the I rules) and
ruff format as a black-compatible formatter.

Tasks

• Add ruff configuration in pyproject.toml: the default rule set plus I (import sorting), with
  target-version matching the supported Python floor.
• Run ruff check --fix and ruff format; keep the mechanical changes in their own commit,
  separate from any manual fixes they surface.
• Replace black with ruff in the Pipfile/dev tooling, and add a CI step under
  .github/workflows/ running ruff check and ruff format --check.
• Update CONTRIBUTING.md with the new commands.

This also covers import sorting, which was previously tracked as its own issue.