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

includes/class-pr-core-deactivator.php

@@ -1,6 +1,18 @@
 <?php
+/**
+ * Deactivator.
+ *
+ * @package Peptide_Repo_Core
+ */
+
 declare(strict_types=1);
 
+/**
+ * Flushes rewrite rules on deactivation (data preserved)
+ *
+ * @package Peptide_Repo_Core
+ */
+
 /**
  * Plugin deactivation handler.
  *