ci: add lint + unit test workflow

[Mathos34]

Objective

Wire ruff + pytest into GitHub Actions so every push and PR gets a green check. Closes the CI wave on the 5 featured projects.

Changes

• New .github/workflows/ci.yml: single job, ubuntu-latest, 15-minute timeout.
  • Python 3.11 with pip cache.
  • Installs the deps the test path needs: ruff, pytest, CPU torch (via the pytorch download index for the small wheel), and numpy.
  • Skips torchvision, matplotlib, tqdm: not used by tests/.
  • ruff check . then pytest -q tests/.

Validation

• python -m ruff check . locally: clean.
• python -m pytest -q tests/ locally: 14 tests pass.
• YAML inspected by hand; same pip-index split pattern as gridworld PR ci: add lint + unit test workflow #3 and spoken-digits-asr PR ci: add lint + unit test workflow #3.

Risk

low. Pure additive workflow file. No source modification.

Out of scope

• Installing torchvision in CI: only needed for the MNIST/CIFAR loaders in train.py, not for the unit tests.
• Matrix over multiple Python versions.
• README CI badge (separate follow-up PR after this workflow has a green run on main).

Tracked item: cnn-compression-lab CI workflow.