refactor(fleet): #542 review residue — drop dead proxyOptions local, retire stale compliance defer line

The proxyOptions local in WithOtOpcUaControlPlaneSingletons was already unused on master
(pre-existing dead code, not orphaned by the deletion commit — every registration passes
singletonOptions); removed while the file is open. The phase-6-1 compliance script still
Assert-Deferred'd the FleetStatusHub /hosts follow-up — an item that is now deleted, not
deferred — replaced with a retirement comment.

Claude-Session: https://claude.ai/code/session_014WNM4vjoVksyyBraTXSZE1

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/ServiceCollectionExtensions.cs

@@ -50,7 +50,6 @@ public static class ServiceCollectionExtensions
     public static AkkaConfigurationBuilder WithOtOpcUaControlPlaneSingletons(this AkkaConfigurationBuilder builder)
     {
         var singletonOptions = new ClusterSingletonOptions { Role = AdminRole };
-        var proxyOptions = new ClusterSingletonOptions { Role = AdminRole };
 
         // Per-cluster mesh Phase 2: the central end of the central↔node command boundary.
         //