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/interfaces/interface-related-posts-provider.php

@@ -1,6 +1,18 @@
 <?php
+/**
+ * Related Posts Provider.
+ *
+ * @package Peptide_Repo_Core
+ */
+
 declare(strict_types=1);
 
+/**
+ * Defines the contract for retrieving posts related to a given peptide
+ *
+ * @package Peptide_Repo_Core
+ */
+
 /**
  * Interface for fetching related posts for a peptide.
  *