Add local testing instructions and GitHub Actions CI to run pytest

[WhatsYourWhy]

Motivation

• Make it explicit how to run the test suite locally and ensure tests can import the determinant package (editable install or PYTHONPATH fallback).
• Add automated CI to run the same install+test steps on pushes and pull requests to catch import and test regressions early.

Description

• Add a ## Testing section to README.md documenting python -m pip install -e . plus python -m pytest -q and an alternative PYTHONPATH=. python -m pytest -q invocation.
• Add a GitHub Actions workflow at .github/workflows/ci.yml that runs on push and pull_request, sets up Python 3.11, installs the package in editable mode and pytest, and runs python -m pytest -q.

Testing

• No automated tests were executed locally as part of this change; the edits are documentation and CI configuration only.
• The added CI workflow will run python -m pip install -e . pytest and then python -m pytest -q on GitHub Actions (Ubuntu, Python 3.11) for each push and pull request.

---

Codex Task