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>

.githooks/pre-push

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+#
+# Pre-push gate: run the local CI checks (ruff + mypy + pytest) before allowing
+# a push. Blocks the push on any failure.
+#
+# Enable in a fresh clone with:
+#   git config core.hooksPath .githooks
+# Bypass a single push with:
+#   git push --no-verify
+#
+exec "$(git rev-parse --show-toplevel)/scripts/check.sh"