Add a local verification harness and CI workflow

[exploring-curiosity]

This repository has no automated way to check that a change is correct, so no
change to it can be verified before merging.

Add a pyproject.toml dev-dependency group with pytest and ruff, a tests/ directory with at least one real test of existing behaviour, and a ruff.toml or [tool.ruff] section.

Also add .github/workflows/ci.yml running the same commands on push and
pull_request.

Requirements:

• Every command must run locally with no API keys, no cloud services, and no GPU.
• Tests must assert real behaviour of existing code. A test that only asserts
  True == True does not satisfy this issue.
• Do not change existing application logic to make tests pass.