ci: add local pre-push gate (ruff + mypy + pytest)

Forgejo Actions is not enabled on the instance, so run the same checks
locally before pushing:

- scripts/check.sh runs ruff, mypy, and pytest (mirrors the CI workflow).
- .githooks/pre-push invokes it; enable per clone with
  `git config core.hooksPath .githooks`. Bypass with `git push --no-verify`.
- Fix the pytest invocation in the CI workflow (and document it): use
  `python -m pytest` so the repo root is on sys.path, otherwise the test
  modules fail to `import tests.*`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -42,4 +42,6 @@ jobs:
         run: pip install -e ".[dev]"
 
       - name: Run test suite
-        run: pytest -q
+        # `python -m pytest` puts the repo root on sys.path so the test modules
+        # can `import tests.*` (the pytest console script would not).
+        run: python -m pytest -q