fix(security): content-create divi follows the Divi group switches (audit M2)

The divi input of lit/content-create is refused with unsupported when
lit/divi-insert is disabled or the Divi group is disabled or read-only
(ExposurePolicy::allows as a cross-group gate, asked at execute time).
content-create without divi keeps working. Adds a registrar test that
read-only write tools are never registered, so execute-ability can't
reach them either.

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

src/Integrations/Divi/DiviIntegration.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi;
 
+use Closure;
 use LIT\MCP\ChangeLog\ChangeLog;
 use LIT\MCP\Integrations\Divi\Design\DesignSystemReader;
 use LIT\MCP\Integrations\Divi\Design\DiviDesignSystemAbility;
@@ -58,14 +59,20 @@ final class DiviIntegration implements Integration
         return DiviModules::isAvailable();
     }
 
-    /** Divi short form for lit/content-create; available() reports whether Divi 5 is loaded. */
-    public function contentComposer(): ContentComposer
+    /**
+     * Divi short form for lit/content-create; available() reports whether Divi 5 is loaded,
+     * writable() asks $writable (the Tools-tab gate, security M2).
+     *
+     * @param (Closure(): bool)|null $writable
+     */
+    public function contentComposer(?Closure $writable = null): ContentComposer
     {
         return new DiviContentComposer(
             $this->schema,
             new DiviComposition($this->modules, $this->design),
             new DiviSaver(),
-            $this->design
+            $this->design,
+            $writable
         );
     }