Refresh CI lint coverage

[juliosuas]

Summary

• include conftest.py in Ruff CI checks
• apply Ruff formatting to conftest.py
• update GitHub Actions to checkout v6 and setup-python v6 to avoid Node 20 runner deprecation warnings

Verification

• .venv/bin/python -m ruff check ghost tests conftest.py
• .venv/bin/python -m ruff format --check ghost tests conftest.py
• .venv/bin/python -m pytest -q