Delete contact

Permanently delete a contact and its communication channels. Implements right-to-erasure obligations under GDPR Art. 17, Ley 19.628 (Chile) Art. 12, CCPA § 1798.105, and LGPD Art. 18.VI. The contact, its channels, and any associated agent flow sessions and conversation threads are removed. Past message records and broadcast delivery logs are retained for billing/audit but no longer reference the deleted contact.