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/Pages/DiviContentComposer.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi\Pages;
 
+use Closure;
 use LIT\MCP\Abilities\Errors;
 use LIT\MCP\Integrations\Divi\BlockTree\Block;
 use LIT\MCP\Integrations\Divi\BlockTree\Document;
@@ -34,12 +35,18 @@ use WP_Error;
  */
 final class DiviContentComposer implements ContentComposer
 {
+    /** @var Closure(): bool */
+    private Closure $writable;
+
+    /** @param (Closure(): bool)|null $writable Tools-tab gate (Plugin: lit/divi-insert enabled); null = always. */
     public function __construct(
         private ContentSchema $schema,
         private DiviComposition $composition,
         private DiviSaver $saver,
-        private DesignSystemReader $design
+        private DesignSystemReader $design,
+        ?Closure $writable = null
     ) {
+        $this->writable = $writable ?? static fn (): bool => true;
     }
 
     public function available(): bool
@@ -47,6 +54,11 @@ final class DiviContentComposer implements ContentComposer
         return DiviModules::isAvailable();
     }
 
+    public function writable(): bool
+    {
+        return ($this->writable)();
+    }
+
     public function check(string $postType, array $sections): ?WP_Error
     {
         if (! $this->schema->diviEnabled($postType)) {