Add GitHub Actions workflow for running tests

[malandrina]

Why?

It can be hard to remember to run tests locally before merging. Running tests in a CI pipeline will give everyone more confidence in the code we're shipping.

How?

• I asked Aider to generate a GitHub Actions workflow for running pytest tests on CI using uv
• Then I modified the file it generated to only use Python 3.13 instead of Python 3.10, 3.11 and 3.12
• I also updated a test that requires the presence of an empty directory to create and tear down the empty directory. This is necessary because git does not track empty directories.

Testing

• Example failed run: https://github.com/nuanced-dev/nuanced/actions/runs/15911654804/job/44880259316
• Example successful run: https://github.com/nuanced-dev/nuanced/actions/runs/15911670367/job/44880312446