refactor(skills): rename page-structure skill to designer-tools and update descriptions

Rename skill folder and frontmatter from page-structure to designer-tools
to reflect the broader scope (pages, elements, components, styles).
Update Cursor marketplace description accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yan-xie-nk

.cursor-plugin/marketplace.json

@@ -7,7 +7,7 @@
     {
       "name": "webflow",
       "source": "./",
-      "description": "Production-ready agent skills for Webflow - CMS management, site auditing, asset optimization, safe publishing, and Designer page structure tools."
+      "description": "Production-ready agent skills for Webflow - CMS management, site auditing, asset optimization, safe publishing, and Designer tools for building pages, elements, components, and styles."
     }
   ]
 }