Add a script to mirror main to GitHub after the move to Gitea

Werkator's canonical repository now lives on git.javagil.de; GitHub stays
around as a public mirror. This keeps its main branch in sync without
touching any other branch or the repo's other history.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/push-github-mirror.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+#
+# Push the current main branch to the GitHub mirror.
+#
+# Werkator's canonical repository lives on git.javagil.de (Gitea, remote
+# "origin"); GitHub stays around as a public read-only mirror (remote
+# "github"). This script keeps that mirror's main branch in sync — nothing
+# else: no other branches, no force-push, no tags. Run it after pushing main
+# to origin, or set it up as a post-push hook / cron job if that becomes
+# annoying to remember.
+#
+# Usage: tools/push-github-mirror.sh
+
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+git fetch origin main
+git push github refs/remotes/origin/main:refs/heads/main