docs(specs): record xce-node-manager interactions with service and provisioning specs

xce-node-manager: a "Cross-Spec Interactions" section (pending a
requirements revision; approved criteria unchanged) with six points: a
supervision/probe module shared with service-supervision, node-manager
components vs service jobs, a lease-based provisioner role, role
declaration and worker stop on decommission for provisioned hosts, a
host-loss grace period longer than worker restart plus rejoin, and a future
service proxy under the reverse-proxy role. design.md Open Questions points
to it.

The service-supervision, host-failure-job-recovery, elastic-hosts,
resource-provisioner and service-endpoint briefs cross-reference the
matching points; the roadmap lists the xce-node-manager revision.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.kiro/specs/elastic-hosts/brief.md

@@ -63,7 +63,12 @@ provisioned hosts; template-based host config; a decommission control.
 ## Existing Spec Touchpoints
 - **Extends**: none.
 - **Adjacent**: `worker-join-refactor`, `peer-authentication-hardening`,
-  `xce-node-manager`.
+  `xce-node-manager`: a provisioned host boots with the Node Manager and the
+  `worker` role, declared by its host template and written into the Node
+  Manager's configuration by the instance's boot script; on decommission
+  the Node Manager stops the worker (or drops its role) instead of
+  restarting it, and the Controller refuses a rejoin from a decommissioned
+  host (`xce-node-manager` requirements.md, "Cross-Spec Interactions" 4).
 
 ## Constraints
 - HA/persistence: full 3-phase workflow; the scheduler must see the same