fix: remove re-introduced skills array from marketplace.json, bump plugin to v1.9.0

PR #264 was based on pre-fix main and re-introduced the inline skills
array that #273 removed. plugin.json with "skills": "./skills" is
the authority.

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

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "marketing-skills",
   "description": "Marketing skills for AI agents — conversion optimization, copywriting, SEO, paid ads, ad creative, and growth",
-  "version": "1.8.0",
+  "version": "1.9.0",
   "author": {
     "name": "Corey Haines"
   },