Add pytest and Python linting (ruff) to CI

[mnriem]

Problem

Tests exist (tests/test_extensions. ~39 tests), pytest is configured in pyproject.toml, and test deps are but **no workflow runs them**. The lint workflow only runs markdownlint-cli2. A contributor can break the CLI or extension system and CI still passes green.declared py

Proposed Solution

Add a new .github/workflows/test.yml workflow that runs on every push to main and on PRs:

• ruff check src/ and ruff format --check src/ for Python linting/formatting
• uv run pytest across Python 3.11, 3.12, and 3.13 matrix

This keeps Python CI concerns separate from the existing markdown lint workflow.