ci: resolve the Shopware version with Composer from the plugin constraint; harden the registry repack

- phpunit: 'Resolve the latest Shopware Version' uses composer show against require.shopware/core
  from the plugin's composer.json (the Packagist scrape returned nothing on the first run)
- release: build the registry zip from an explicit file list without a leading ./ so the
  composer.json entry is stored at the zip root

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -53,10 +53,10 @@ activates this plugin inside a real Shopware kernel for the test run (`KERNEL_CL
 `Shopware\Core\Kernel` in `phpunit.xml`). This means tests require a working Shopware installation/DB
 to bootstrap against — they are not runnable as an isolated PHP package.
 
-In CI (`phpunit` job) the latest Shopware 6.6 version is resolved from Packagist and the prepared project
-(`composer create-project` + phpunit, then `system:install` into `shopware_test` on a MariaDB 10.11 service)
-plus a SQL dump of the installed database are cached under a key that contains that version, so a new
-Shopware release invalidates the cache. On a hit the dump is restored instead of reinstalling; the plugin
+In CI (`phpunit` job) the latest Shopware version allowed by the plugin's `require.shopware/core` constraint
+is resolved with Composer. The prepared project (`composer create-project` + phpunit, then `system:install`
+into `shopware_test` on a MariaDB 10.11 service) plus a SQL dump of the installed database are cached under
+a key that contains that version, so a new Shopware release invalidates the cache. On a hit the dump is restored instead of reinstalling; the plugin
 is then copied into `custom/plugins` and the bootstrapper only activates it.
 
 ```bash