feat: add churn-prevention skill for cancel flows, save offers, and dunning

Covers voluntary churn (exit surveys, dynamic save offers, pause/downgrade,
cancel flow UI patterns) and involuntary churn (smart retries, dunning emails,
card updaters, grace periods). Includes Churnkey-style offer-to-reason mapping,
health scoring model, proactive retention triggers, and provider-specific
dunning setup for Stripe, Chargebee, Paddle, and Recurly.

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

.claude-plugin/marketplace.json

@@ -12,13 +12,14 @@
   "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, paid ads, ad creative, churn prevention, pricing strategy, referral programs, and more",
       "source": "./",
       "strict": false,
       "skills": [
         "./skills/ab-test-setup",
         "./skills/ad-creative",
         "./skills/analytics-tracking",
+        "./skills/churn-prevention",
         "./skills/cold-email",
         "./skills/competitor-alternatives",
         "./skills/content-strategy",