Add fleet groups, agent screenshots, deploy guards, and Crucible polish.

server/internal/api/fleet_handler.go

@@ -310,8 +310,22 @@ func (f *FleetHandler) PostAgentCommand(w http.ResponseWriter, r *http.Request)
 		}
 		f.ws.BroadcastAgentCommand(req.Action, args)
 	} else {
+		if !f.ws.isAgentConnected(id) {
+			writeJSON(w, map[string]interface{}{
+				"success":  false,
+				"error":    "agent not connected",
+				"agent_id": id,
+				"action":   req.Action,
+			})
+			return
+		}
 		if err := f.ws.SendAgentCommand(id, req.Action, args); err != nil {
-			http.Error(w, err.Error(), http.StatusBadRequest)
+			writeJSON(w, map[string]interface{}{
+				"success":  false,
+				"error":    err.Error(),
+				"agent_id": id,
+				"action":   req.Action,
+			})
 			return
 		}
 	}