Add wp-env as the default local test environment

[remyperona]

What

Introduces @wordpress/env as the default, supported way to run this package's unit and WordPress integration tests locally.

wp-env provides a Dockerized WordPress + MySQL with Composer, PHPUnit, and WP-CLI preinstalled, and exposes the WordPress test suite via WP_TESTS_DIR — which the existing integration bootstrap (src/Integration/bootstrap.php) already consumes through Yoast's get_path_to_wp_test_dir(). No production code changes were needed.

Changes

• .wp-env.json (new) — latest WordPress, PHP 8.3, repo mounted via mappings into wp-content/plugins/wpmedia-phpunit. It's a library (no plugin header), so it is not listed under plugins to avoid an activation failure. Ports set to 8890/8891 because wp-env's defaults 8888/8889 collide with Local by Flywheel.
• package.json / package-lock.json (new) — @wordpress/env devDependency plus env:* and test:php* wrapper scripts.
• README.md — new "Running This Package's Tests" section documenting wp-env as the default path.
• CLAUDE.md (new) — agent guidance covering the wp-env workflow, ports, library-not-plugin mounting, and the CI-vs-local split.
• .gitignore — ignore node_modules/ and .wp-env.override.json.

Developer flow

```bash
npm install && npm run env:start && npm run env:install
npm run test:php
```

Scope / CI

This is the local default only. CI is unchanged: tests.yml still runs the PHP 8.0–8.5 matrix and tests_legacy.yml runs PHP 7.4 / WP 5.9 via the wp-media/workflows/setup-wp-tests action (wp-env can't cover that full matrix in one config). Both wp-env and CI invoke the same composer scripts.

Verification

Ran end to end via wp-env on PHP 8.3.33 — all green:

• Unit: 101 tests, 303 assertions — OK
• Integration: 29 tests, 70 assertions — OK
• Admin integration: 3 tests, 4 assertions — OK

🤖 Generated with Claude Code