Adds adapter-owned externalEventId extraction to the ChannelAdapter contract
(WebhookAdapter→eventId, EmailAdapter→Message-ID) and uses it in InboundService,
so email replays dedup on Message-ID (previously only payload.eventId worked).
Adds an email-dedup spec + smoke-email.mjs proving inbound → normalized, a reply
joins the same email thread, bad signature rejected, and outbound EMAIL SENT.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
