feat: add empty client spool reconciliation

internal/server/session/agent_server.go

@@ -184,6 +184,22 @@ func (server *AgentServer) serveConnection(parent context.Context, connection *w
 			server.close(connection, websocket.StatusPolicyViolation, ErrStaleSession.Error())
 			return
 		}
+		if snapshot := envelope.GetReconcileSnapshot(); snapshot != nil {
+			// A retained command requires the command-store reconciliation phase,
+			// which is deliberately not substituted with a no-op result.
+			if len(snapshot.GetRetainedCommands()) != 0 {
+				server.close(connection, websocket.StatusPolicyViolation, "reconciliation for retained commands is unavailable")
+				return
+			}
+			encoded, err := proto.Marshal(&rvboxv1.AgentEnvelope{
+				SessionId: encodedSessionID, SessionGeneration: registration.Generation,
+				Payload: &rvboxv1.AgentEnvelope_ReconcileResult{ReconcileResult: &rvboxv1.ReconcileResult{}},
+			})
+			if err != nil || queue.EnqueueControl(Frame{Kind: FrameControl, Payload: encoded}) != nil {
+				server.close(connection, websocket.StatusInternalError, "could not queue reconciliation result")
+				return
+			}
+		}
 	}
 }