Adds a canonical EmailInboundPayload + EmailAdapter implementing the
ChannelAdapter contract: HMAC signature verify + normalize with email-native
threading (whole reply chain → one IIOS thread via references/inReplyTo root),
messageId as providerEventId for dedup, html→text fallback, Re:/Fwd: stripping.
Plus emailInboundFixture/emailReplyFixture and unit tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
