Add reproducible WordPress integration test runner

[jt55401]

Summary

• add a digest-pinned PHP and MariaDB Docker test environment
• run the 55-test WordPress PHPUnit suite with one command
• document runtime and test-version boundaries
• add the passing Docker test path to CI
• keep the existing PHPCS baseline available through an explicit --lint mode

Validation

• ./wordpress/bin/test-docker.sh (55 tests, 226 assertions)
• bash -n wordpress/bin/test-docker.sh wordpress/bin/run-docker-tests.sh
• docker compose -f wordpress/docker-compose.test.yml config --quiet
• actionlint .github/workflows/ci.yml
• git diff --check

./wordpress/bin/test-docker.sh --lint reaches PHPCS after PHPUnit and reports the existing baseline of 10,633 errors and 442 warnings. The default CI path runs PHPUnit.