fix(security): no direct HTTP access to plugin PHP files (audit L2, cheap part)

Every file under src/ exits outside WordPress (defined('ABSPATH') ||
exit; after the use block, so a direct request no longer prints a fatal
error with the server path); divi-convert-fixture.php gets the WP_CLI
guard the other integration scripts have. A unit test keeps both true;
PSR1 side-effects warning excluded for the guard.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Abilities/AbilityRegistrar.php

@@ -11,6 +11,8 @@ use LIT\MCP\Server\RequestContext;
 use Throwable;
 use WP_Error;
 
+defined('ABSPATH') || exit;
+
 /**
  * Registers all abilities of available integrations with the Abilities API and
  * wraps them with the role gate and neutral error handling.