ci: add github actions workflow

[X0luka]

Summary

• Add the initial GitHub Actions CI workflow required by master branch protection
• Run uv dependency sync, ruff, pytest, and golden set JSON validation
• Keep this PR infrastructure-only so future PRs can produce the required CI check automatically

Validation

• uv sync --extra dev
• uv run ruff check src ui eval/run_eval.py scripts tests
• uv run pytest tests/test_chunker.py tests/test_bm25.py tests/test_hybrid.py -v
• uv run python -m json.tool eval/golden_set.json >/dev/null

Notes

• The workflow intentionally does not reference eval/generate_golden_set.py because that file is not on current master.
• This bootstrap PR may require an admin bypass or temporary branch protection adjustment because master already requires the CI check that this PR introduces.