Enhancement: Run friendsofphp/php-cs-fixer on GitHub Actions

[lacatoire]

This pull request introduces a new GitHub Actions workflow to automate basic CI tasks for the PHD project.

What’s included:

• A workflow file named integrate.yml that runs on every push and pull request.
• Coding Standards: runs PHP 8.1 with the required extensions (dom, xmlreader, sqlite3) and executes PHP-CS-Fixer (rules currently empty, ready for community discussion).

Next steps (for future PRs):

• Define and agree on a common PHP-CS-Fixer ruleset (.php-cs-fixer.php).
• Extend CI to cover different PHP versions if needed.
• Add caching or build artifacts when the project evolves.

This mirrors the approach used in php/web-php#559