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/Guides/GuideRepository.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Guides;
 
+defined('ABSPATH') || exit;
+
 /**
  * Guide topics: markdown files from one directory (the default), extended or overridden
  * by the site via the filter `lit_mcp_guides` (spec §9.2), which returns

src/Guides/InstructionsBuilder.php

@@ -8,6 +8,8 @@ use LIT\MCP\Abilities\ExposurePolicy;
 use LIT\MCP\Integrations\Integration;
 use LIT\MCP\Integrations\IntegrationRegistry;
 
+defined('ABSPATH') || exit;
+
 /**
  * Builds the short server instructions every MCP client loads on connect.
  * Keep this text short: it occupies context in every conversation.