Add relay_instance_id() (env GATEWAY_RELAY_INSTANCE_ID first, then
gateway.relay_instance_id in config.yaml, mirroring the other relay readers) and
forward it in the /relay/provision body so the connector can bind
gatewayId -> instanceId and route inbound per-instance once Phase 6 delivery
lands.
The value is gateway-asserted but safely scoped: the org/tenant stays
NAS-token-verified at the connector, so a dishonest gateway can only bind its
OWN tenant's instance — same posture as relay_endpoint(). instanceId is only
added to the body when present, so omitting it lets the connector store null
(back-compat: self-hosted / pre-Phase-6 gateways simply have no binding yet).
For a managed (NAS-hosted) agent the id is NAS's AgentInstance.id, stamped into
the container env beside GATEWAY_RELAY_URL.
Tests: reader (env/config/absent), self_provision_relay forwards the id (set +
absent), and the real _post_provision body includes instanceId ONLY when set.
Refs: ~/nous/specs/gateway-gateway plan.md Phase 6 Unit α; decisions.md Q11.
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