Architecture v3: pluggable MailBackend; competitive positioning; relay-first

- Confirm stack: Go, single binary, embed Mox + pluggable backend (chosen after
  landscape research).
- Add MailBackend interface: relay (v1 default) / imap_smtp (BYO mailbox) /
  embedded (go-smtp + mox smtpclient/dane/mtasts, flagship). Deliverability
  becomes opt-in; useful on day 1.
- Add competitive positioning (§0.1): closest competitor agenticmail is TS +
  Stalwart(AGPL) Docker sidecar; OpenMail differentiates as single binary,
  all-MIT, in-process. agentic-inbox is Cloudflare-locked.
- Reorder milestones backend-first: relay loop before embedded SMTP.
- State scope: self-hostable app, not a SaaS — no billing/Stripe.
- README: pluggable backends + positioning. Repo layout: internal/mail/*.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -9,6 +9,12 @@ internals of [Mox](https://github.com/mjl-/mox) (also MIT) for the hard, correct
 plumbing — DKIM, SPF/DMARC, DANE + MTA-STS secure delivery, real-world MIME parsing, spam
 filtering — and layers a native, agent-shaped data model (Postgres + object storage) and API on top.
 
+Mail I/O is **pluggable**: start in minutes against a relay (SES/Postmark/Resend) or your existing
+mailbox (IMAP/SMTP), and graduate to a fully self-hosted, in-process SMTP engine when you want to own
+the whole stack. The in-process engine is the part nobody else ships — the only comparable project,
+[agenticmail](https://github.com/agenticmail/agenticmail), runs a Stalwart (AGPL) mail server in a
+Docker sidecar; OpenMail is **one static binary, fully MIT, no sidecar.**
+
 > **Status: early WIP, private during initial build.** Will be released MIT-licensed and public.
 > Designed only from public RFCs and public API surfaces — nothing proprietary.