flat_dm_status was speculative config (rubric violation): no user wants
'make my agent look dead', and the only real consumer of status
suppression was native's placement-contamination guard — which the relay
lane handles structurally (QA-6/7 send-side anchor strip, leak-guard
test), not via preference. Status now anchors whenever an inbound ts
exists, in both modes.
Docs: new website/docs/user-guide/messaging/relay.md — enterprise-only
relay lane page documenting the platforms.relay.extra.<platform> subset
shape (nested wins, flat fallback), the Slack reply_in_thread control,
and always-on liveliness. Kept out of the native slack.md on purpose:
relay controls are not Slack config.