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

tests/Unit/Integrations/Divi/Pages/DiviContentComposerTest.php

@@ -142,4 +142,19 @@ final class DiviContentComposerTest extends TestCase
         $this->expectException(RuntimeException::class);
         $this->composer()->write(42, 'page', self::sections());
     }
+
+    public function testWritableFollowsTheGateAndDefaultsToTrue(): void
+    {
+        self::assertTrue($this->composer()->writable());
+
+        $design = DiviWriterFixture::designReader();
+        $gated = new DiviContentComposer(
+            DiviFixtures::schema(true),
+            DiviWriterFixture::composition($design),
+            $this->saver,
+            $design,
+            static fn (): bool => false
+        );
+        self::assertFalse($gated->writable());
+    }
 }