Initial email matrix and infra pattern

Summary

  • Separated human / org email from product messaging contexts.

  • Locked de-facto lanes: ZeptoMail (human send-as), Resend (app/code only), Customer.io (journeys), Cloudflare Email Routing (inbound).

  • Documented DMARC p=none → quarantine → reject.

  • Documented Resend subdomain roles (updates / send / links).

  • Documented org-level Pulumi infra repo pattern with per-repo Antora docs.