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>

skills/email-sequence/SKILL.md

@@ -301,6 +301,7 @@ For implementation, see the [tools registry](../../tools/REGISTRY.md). Key email
 
 ## Related Skills
 
+- **churn-prevention**: For cancel flows, save offers, and dunning strategy (email supports this)
 - **onboarding-cro**: For in-app onboarding (email supports this)
 - **copywriting**: For landing pages emails link to
 - **ab-test-setup**: For testing email elements