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>
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