ci: run ruff and pytest on pushes and pull requests

[73anthonyL]

What

Adds a GitHub Actions workflow, .github/workflows/ci.yml, that runs on pushes to main and on every pull request:

• ruff check .
• ruff format --check .
• pytest -q

PR #1 merged with no checks; this is the first PR that will show them.

Choices

• Python 3.11 only. It is the supported floor, and the development venv is 3.12, so 3.11 is the version nothing else exercises.
• CPU-only torch installed before the package, so sentence-transformers does not pull the CUDA wheels onto the runner.
• No models are downloaded and no secrets are needed: the suite stubs the embedder and generates its own PDFs.

All three are recorded in DECISIONS.md under "Continuous integration".

Verification

• Locally on this branch: ruff check and ruff format --check clean.
• The workflow itself has not run before; this PR's own check run is its first test.