f0afca89e3
AdapterRegistry now backs the EMAIL channel with the email-native EmailAdapter
(RFC threading) instead of the generic WebhookAdapter. Adds EmailProvider — an
email-envelope egress provider ({to,subject,text,html,inReplyTo}) registered for
EMAIL when IIOS_PROVIDER_URL_EMAIL is set (sandbox stays the default). Spec:
signed email → normalized interaction on the email thread; a reply joins the
same thread (one conversation); bad signature → rejected (fail-closed).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>