Repair wp/5.6 branch Actions: Run PHP coding standards on the runner instead of inside wp-env - #84964

Part of #82937.

What?

Moves PHP coding standards checks (phpcs) out of wp-env into a new phpcs job in unit-test.yml. The job runs on the runner at PHP 7.4, reports violations as annotations with cs2pr, and checks that no version-controlled files changed. It passes --runtime-set ignore_warnings_on_exit 1, as this branch's composer run-script lint does. The .cache/.gitkeep, .gitignore and phpcs.xml.dist changes add the PHPCS scan cache.

The single site PHPUnit leg now runs npm run test-unit-php instead of npm run test-php, which ran lint-php first. The pretest-unit-php and pretest-unit-php-multisite hooks now run composer install after wp-env start.

Why?

Coding standards checks don't depend on the PHP version being tested. Inside wp-env they only ran after the environment started, they repeated on every leg of the PHP matrix added in #84963, and violations showed up as container logs instead of annotations on the diff.

lint-php's prelint-php hook was the only thing that installed Composer dependencies for PHPUnit, and phpunit/bootstrap.php requires vendor/autoload.php. Moving the install into the pretest hooks keeps it, and also gives the multisite leg the install it never had.

The job uses PHP 7.4 because the PHP_CodeSniffer and WPCS releases this branch depends on emit deprecation notices on PHP 8.4 and newer that abort the scan.

How?

Ports the phpcs job from #46510 (9ee17a7).

The phpcs job's composer install fails until #84965, directly above, adds config.allow-plugins. With #84965 applied, phpcs passes.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

🤖 Generated with Claude Code