add CI workflow and clean up lint/type checks

[fwph]

Summary

• add a GitHub Actions workflow for Poetry-based CI
• add the Poetry guard plugin wiring and supporting ignore rules
• clean up black, flake8, and mypy issues so the repo checks pass cleanly
• add a public-repo checklist for follow-up cleanup work

Validation

• poetry run black splat tests
• poetry run flake8 splat tests
• poetry run mypy splat
• poetry run pytest -q