Add CI workflow for tests and linting

[mzargham]

Summary

The project has pytest tests and ruff configuration but no GitHub Actions CI workflow. The .github/workflows/ directory does not exist.

Suggested workflow

A basic CI workflow that runs on push/PR to main:

1. uv sync — install dependencies
2. uv run ruff check symproof/ — linting
3. uv run ruff format --check symproof/ — format check
4. uv run python -m pytest tests/ -v — test suite

Target Python 3.12+ (per requires-python in pyproject.toml).

Why this matters

For a proof engine, automated regression testing is especially important — mathematical correctness bugs (see issues #1, #2) need to be caught by CI, not discovered in production.