GitHub Actions: pytest across Python 3.10-3.13, a clean-install import smoke
that catches undeclared runtime deps, and a strict Ruff gate (standard rule set)
scoped to the files each PR changes. Declares python-dotenv (imported by the CLI
but previously undeclared) and adds a [dev] extra. Recommends Python 3.12 for
setup, verified from a clean isolated install.
