add CLAUDE.md

- CLAUDE.md: document project architecture, resilience constraint,
  workflow and all helper scripts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pool.js

@@ -299,14 +299,13 @@ MQTT.subscribe(
   function (topic, msg) {
     status.tick_mqtt++;
     print("[POOL] mqtt", topic, msg);
-    if (msg == "") {
-      return;
+    if (msg != "") {
+      let obj = JSON.parse(msg);
+      if (obj.temperature === undefined) {
+        return;
+      }
+      update_temp(obj.temperature);
     }
-    let obj = JSON.parse(msg);
-    if (obj.temperature === undefined) {
-      return;
-    }
-    update_temp(obj.temperature);
   }
 )