Run the local gate from a versioned pre-push hook

The Gitea server has no Actions, so scripts/check.sh is the gate. A
versioned .githooks/pre-push runs it; CONTRIBUTING explains how to
enable it per clone and when it may be skipped.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.githooks/pre-push

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Versioned pre-push hook. The server runs no CI, so the local gate runs here.
+# Enable once per clone:   git config core.hooksPath .githooks
+# Skip only deliberately:  git push --no-verify
+set -euo pipefail
+cd "$(git rev-parse --show-toplevel)"
+exec scripts/check.sh

CONTRIBUTING.md

@@ -37,6 +37,17 @@ dependencies are not accepted without prior discussion.
 
 ## Tests
 
+The server runs no CI. The gate is `scripts/check.sh`, run by the versioned
+`pre-push` hook once you enable it in your clone:
+
+```bash
+git config core.hooksPath .githooks
+```
+
+Skip it only deliberately with `git push --no-verify`, and say why in the
+commit message. The workflows in `.gitea/workflows` are ready for a Gitea 1.21
+or later with a runner; until then they do not run.
+
 ```bash
 ./scripts/check.sh                     # everything ci.yml runs, on any machine; run before pushing
 go test -race ./...