feat: add ad-creative skill for bulk ad creative generation and iteration

Inspired by Anthropic growth marketing team workflows. Covers generating
headlines, descriptions, and primary text at scale across Google Ads, Meta,
LinkedIn, TikTok, and Twitter/X with platform character limits, performance
based iteration loops, and batch generation workflows.

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

.claude-plugin/marketplace.json

@@ -12,11 +12,12 @@
   "plugins": [
     {
       "name": "marketing-skills",
-      "description": "26 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, paid ads, pricing strategy, referral programs, and more",
+      "description": "27 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, paid ads, ad creative, pricing strategy, referral programs, and more",
       "source": "./",
       "strict": false,
       "skills": [
         "./skills/ab-test-setup",
+        "./skills/ad-creative",
         "./skills/analytics-tracking",
         "./skills/cold-email",
         "./skills/competitor-alternatives",