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/WordPress/Content/ContentComposer.php

@@ -21,6 +21,12 @@ interface ContentComposer
     /** Whether composing is possible on this site right now (e.g. Divi 5 active). */
     public function available(): bool;
 
+    /**
+     * Whether the site owner lets LIT MCP write such content (Tools tab: for Divi, lit/divi-insert
+     * is not disabled and the Divi group is neither disabled nor "nur lesen" – security M2).
+     */
+    public function writable(): bool;
+
     /**
      * Validates the description for a new entry of $postType. Nothing is written.
      *