feat: detect needed PHP version from composer metadata

[MekDrop]

Summary

• add �in/resolve-php-version.sh for PHP version resolution
• detect PHP requirement from composer.json first (
  equire.php, then config.platform.php)
• fall back to composer.lock (platform.php, then platform-overrides.php)
• keep existing engine defaults (7.4 for evert/phpdoc-md, 8.1 for clean/phpdoc-md) as fallback
• update �in/setup.sh to use the resolver helper

Notes

• resolver normalizes the detected constraint to major.minor for setup-php

Testing

• unable to execute bash scripts in this Windows environment (/bin/bash not available), so runtime validation should happen in CI