Adopt reusable CI standard + PHPUnit conversion (#22)

Squash-merge of PR #22: adopt shared reusable CI workflow, convert PHPUnit to stubs, update branch-protection contexts.

QA-approved at 05d8094a1d. All required CI checks green.

Agent-Session: 2026-06-17-prcore-ci-deploy

composer.json

@@ -10,7 +10,9 @@
 		"squizlabs/php_codesniffer": "^3.13",
 		"wp-coding-standards/wpcs": "^3.0",
 		"phpcsstandards/phpcsutils": "^1.0",
-		"dealerdirect/phpcodesniffer-composer-installer": "^1.0"
+		"dealerdirect/phpcodesniffer-composer-installer": "^1.0",
+		"phpunit/phpunit": "^9.6",
+		"yoast/phpunit-polyfills": "^2.0"
 	},
 	"config": {
 		"allow-plugins": {
@@ -19,6 +21,11 @@
 	},
 	"scripts": {
 		"lint": "phpcs",
-		"lint:fix": "phpcbf"
+		"lint:fix": "phpcbf",
+		"test": "vendor/bin/phpunit --configuration phpunit.xml.dist",
+		"check": [
+			"@lint",
+			"@test"
+		]
 	}
 }