feat: add ai-seo skill for AI search optimization (AEO, GEO, LLMO)

Split AEO/GEO content patterns out of seo-audit into a standalone ai-seo
skill. Covers AI visibility auditing, content extractability, authority
signals, third-party presence strategy, schema markup for AI, monitoring
tools (Otterly, Peec AI, ZipTie), and optimization by content type.

Moves aeo-geo-patterns.md from seo-audit/references to ai-seo/references
(renamed to content-patterns.md). Updates seo-audit to cross-reference
the new skill.

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

.claude-plugin/marketplace.json

@@ -12,12 +12,13 @@
   "plugins": [
     {
       "name": "marketing-skills",
-      "description": "27 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, paid ads, ad creative, pricing strategy, referral programs, and more",
+      "description": "28 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, AI SEO, paid ads, ad creative, pricing strategy, referral programs, and more",
       "source": "./",
       "strict": false,
       "skills": [
         "./skills/ab-test-setup",
         "./skills/ad-creative",
+        "./skills/ai-seo",
         "./skills/analytics-tracking",
         "./skills/cold-email",
         "./skills/competitor-alternatives",