ci(lint): add Ruff lint, coverage gate & Python 3.13 support

[bcdady]

CI Enhancements

Key additions

Python 3.13 support

• Extended test matrix in .github/workflows/ci.yaml
• Added classifier to pyproject.toml

Static linting

• Added ruff ≥0.4 to dev dependencies
• New “Lint with Ruff” step runs before tests

Code-coverage gate

• Tests now run with pytest --cov
• Build fails if overall coverage < 75 % (current is 78 %)
• Coverage reminder notice encourages incremental improvement
  • Contributing guidelines state that we want to maintain test coverage above 80%, so there's a new message in the CI output to encourage contributors that "PRs that raise coverage are welcome!". Once coverage is up over 80, this threshold should be raised back up to (at least) 80

Developer experience

• Dev dependencies now include Ruff for local use (e.g. with Taskfile)

[rafaelherik]

I will abandon this one as the #15 was merged.