docs: rewrite README for v2

Covers RFC 8058 one-click unsubscribe, polymorphic model support with
morph maps, enum mailing lists, the updated handler API, and the new
Subscriber::fake() testing helper. Removes the obsolete userModel section.

https://claude.ai/code/session_01R4pAjWwGY8xKspsdU8xnsy

Author: claude