PHPCS: fix file docblock format (short description + blank line after)

File-level docblocks lacked a short description and required blank line
between the block and declare(strict_types=1). WP PHPCS file comment
sniff requires both. All 21 PHP files corrected.

Agent-Session: pr-vision rename v0.1.1

includes/core/class-prv-activator.php

@@ -1,5 +1,10 @@
 <?php
-/** @package PrVision */
+/**
+ * Plugin activation handler.
+ *
+ * @package PrVision
+ */
+
 declare(strict_types=1);
 
 /**