fix(check-extension): mirror path repos to prevent symlink errors with template files (#218)

Composer installs path repositories as symlinks by default, which causes Magento's template engine to fail when resolving .phtml files. Setting `COMPOSER_MIRROR_PATH_REPOS=1` on all composer install steps forces a copy instead, matching how the package would be installed from Packagist in production.

Adds a .phtml template and an integration test to the demo package that
renders it via Magento's template engine. Without COMPOSER_MIRROR_PATH_REPOS=1
the path repo is installed as a symlink and the test fails; with mirroring
it passes.

Closes #217

_test/demo-package/Test/Integration/TemplateRenderTest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Graycore\DemoPackage\Test\Integration;
+
+use Magento\Framework\View\Element\Template;
+use Magento\TestFramework\Helper\Bootstrap;
+
+/**
+ * Fails when the extension is installed as a symlink (default Composer path repo behavior)
+ * because Magento's template engine cannot resolve .phtml files through symlinks.
+ * Requires COMPOSER_MIRROR_PATH_REPOS=1 during composer install.
+ */
+class TemplateRenderTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @magentoAppArea frontend
+     */
+    public function testTemplateRendersWithoutSymlinkError()
+    {
+        $block = Bootstrap::getObjectManager()->create(Template::class);
+        $block->setTemplate('Graycore_DemoPackage::demo.phtml');
+        $this->assertNotEmpty($block->toHtml());
+    }
+}