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

phpunit.xml.dist

@@ -0,0 +1,20 @@
+<?xml version="1.0"?>
+<phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
+    bootstrap="tests/bootstrap.php"
+    backupGlobals="false"
+    colors="true"
+>
+    <testsuites>
+        <testsuite name="unit">
+            <directory suffix="Test.php">./tests/unit</directory>
+        </testsuite>
+    </testsuites>
+
+    <coverage processUncoveredFiles="true">
+        <include>
+            <directory suffix=".php">./includes</directory>
+        </include>
+    </coverage>
+</phpunit>