[codex] Add Push MD WordPress.org release automation

[artpi]

Summary

• add a WordPress.org SVN deploy script for Push MD release ZIPs
• wire the Push MD GitHub release workflow to deploy stable releases to WordPress.org after uploading the GitHub release asset
• update the WordPress.org readme/title and release docs for the new publish path

Validation

• bash bin/build-plugins.sh
• bash bin/inspect-push-md-zip.sh dist/plugins/push-md.zip
• bash -n bin/deploy-push-md-wporg-svn.sh
• git diff --check
• vendor/bin/phpcs -d memory_limit=1G plugins/push-md
• vendor/bin/phpunit -c phpunit.xml plugins/push-md/Tests/
• node --check plugins/push-md/admin-shell.js
• wp i18n make-pot plugins/push-md /tmp/push-md.pot --domain=push-md --include='*.php,*.js,readme.txt,uninstall.php'
• WordPress.org SVN dry run against dist/plugins/push-md.zip

Notes

The WordPress.org deploy step requires the WPORG_USERNAME and WPORG_PASSWORD repository secrets. It skips prereleases and versions with suffixes such as -beta.