Add CI on Gitea Actions: pytest and ruff on every PR

.gitea/workflows/ci.yaml runs on PRs into main and on pushes to main.
It installs uv, runs `uv sync --locked` with Python 3.12 (the lowest
version requires-python allows), then pytest, `ruff check` and
`ruff format --check`.

ruff is pinned in the dev group (0.17.3). The repo has no ruff config,
so the rules are ruff's defaults for that version; pinning makes CI
and local runs check the same rules. The setup follows the workflow
that already runs green on this Gitea instance (ubuntu-latest, pinned
checkout SHA, uv via the official script). The README gets a CI badge
and the lint commands.

Closes #11

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -14,6 +14,7 @@ dev = [
     "hypothesis>=6.170.14",
     "pytest>=8",
     "pytest-asyncio>=1.0",
+    "ruff==0.17.3",
 ]
 
 [build-system]