Update project documentation for PHP 8.5

[igancev]

Description

Update all documentation files to reflect the PHP 8.5 requirement.

What to do

• Update CLAUDE.md: change PHP Version: 8.4+ to PHP Version: 8.5+
• Update CONTRIBUTING.md: change PHP 8.4+ to PHP 8.5+
• Review README.md for any PHP version mentions

Definition of Done

All documentation references PHP 8.5 as the minimum required version.

Blocked by: #45

Branch strategy: create PR targeting feature/php-8.5, not main.