Split wp-env into dedicated QA and PHPUnit instances

[epeicher]

What it does

Silences wp-env 11's deprecation warning on every wp-env start by migrating off the deprecated dual-environment mode. npm run env:start now brings up only the manual-QA instance (port 8890, 3 containers instead of 6), and PHPUnit runs in a dedicated instance defined by the new .wp-env.tests.json (port 8891), started with npm run env:start:tests.

Rationale

wp-env 11 deprecated starting the dev and tests environments from one config:

⚠ Warning: wp-env starts both development and tests environments by default.
This behavior is deprecated and will be removed in a future version.

The suggested "testsEnvironment": false alone would have broken npm run test:php, which ran inside the tests-cli container that flag removes. The upstream-recommended replacement is a separate config file per environment via the --config flag, which is what this PR does. As a bonus, QA state and test runs no longer share anything, and each stack is half the containers.

Implementation

• .wp-env.json sets "testsEnvironment": false and drops testsPort; it is now purely the QA instance.
• New .wp-env.tests.json defines the PHPUnit instance. It mirrors the old tests environment's WP_DEBUG/SCRIPT_DEBUG false defaults; without that, WordPress 7.0's wp_trigger_error() in wp_update_plugins() fires a notice under the dev-instance default of WP_DEBUG: true that the strict PHPUnit config turns into a test error (caught in pluginsWindowRegistration.php while verifying this migration).
• test:php / test:php:install target the tests instance via wp-env run --config=.wp-env.tests.json cli; its cli container has WP_TESTS_DIR and a generated wp-tests-config.php just like tests-cli did. New env:start:tests / env:stop:tests / env:destroy:tests scripts manage it.
• bin/setup-wp-env.sh drops the tests-cli Gutenberg setup. PHPUnit never needed it (the guidelines test registers its own stub surface), so the tests instance starts with no afterStart at all.
• CI: the php job starts only the tests instance; the generated .wp-env.plugin-check.json also gets "testsEnvironment": false.
• Docs (AGENTS.md, docs/DEVELOPMENT.md, tests/phpunit/bootstrap.php header) updated to the new flow. WP_ENV_TESTS_PORT is obsolete; worktrees now override each instance's port with WP_ENV_PORT per command, or via git-ignored .wp-env.override.json / .wp-env.tests.override.json.

Testing instructions

npm install               # needs @wordpress/env 11.x from the lockfile
npm run env:start:tests   # no deprecation warning, 3 containers
npm run test:php:install
npm run test:php          # 1329 tests, 3496 assertions, green
npm run env:start         # QA instance on :8890, also warning-free

Note: instances created by the old config keep their now-orphaned tests-* containers until you wp-env destroy them once.