Feat: Improved stability + WP core upgrade

[alihm]

• Add cluster-aware WordPress detection to prevent duplicate file copies on multi-node deployments with disk replication
• Upgrade WordPress to 6.9.1-php8.4-fpm
• Clean up README

Changes

Cluster-aware WordPress Detection

On multi-node Flux deployments with /var/www/html disk replication, only the first node should install WordPress files. This PR adds a pre-flight check that:

1. Fetches the app name from http://fluxnode.service:16101/hostinfo
2. Constructs the domain {appName}.app.runonflux.io
3. Checks if WordPress is already running via:
   • HTTP Link header containing wp-json (primary method)
   • wp-login.php returning 200/302 (fallback)
4. Skips copying WordPress files, wp-config.php, and nginx.conf if WordPress is detected

This prevents race conditions and file conflicts when multiple nodes start simultaneously.