Document external UniMail order email integrations

[dt2patel]

Summary

• add an external-partner quickstart for order transactional email events where the partner hosts the HTTPS receiver and does not deploy to Unigate
• document the current /rest/s1/unigate/communication/email contract, canonical messageData, event-scoped items versus full-order allItems, delivery semantics, and certification checklist
• reconcile UniMail onboarding, routing, and CommGatewayAuth docs with the current Unigate and OMS source
• label the existing provider implementation guide as the internal in-process adapter path
• include a current Omnisend Events API mapping example

Runtime boundary

CommGatewayConfig can select only a Moqui service that is already deployed. Configuration alone cannot call an arbitrary partner URL. Before an external partner is certified, HotWax must enable a provider-neutral outbound adapter in the target environment; the partner should not fork or deploy Unigate.

Source reconciliation

• hotwax/hotwax-unigate at 4531383a25240cac5793a618f64caed32aa62551: REST route, service interface, routing errors, and current CommGatewayAuth entity
• hotwax/oms at 32276abc3f87e9333419c73e61b92b43976cffe7: canonical order payload assembly, items/allItems, API-root path handling, retries, and nested downstream status handling
• Omnisend official Events API, events overview, and authentication documentation

Validation

• git diff --check
• pnpm dlx markdownlint-cli@0.45.0 --disable MD013 -- <7 changed Markdown files>
• parsed all 7 JSON examples
• resolved all 65 local Markdown links in the changed files