fix(reliability): engram write

chat.el

@@ -1072,7 +1072,7 @@ fn handle_chat_agentic(body: String) -> String {
     let session_valid: Bool = if str_eq(req_session, "") {
         true
     } else {
-        session_exists(req_session)
+        !str_contains(session_get(req_session), "\"error\"")
     }
     if !session_valid {
         return "{\"error\":\"session not found\",\"session_id\":\"" + req_session + "\",\"reply\":\"\"}"
@@ -1699,6 +1699,8 @@ fn auto_persist(req: String, resp: String) -> Void {
             "session_bell_signal:" + sess_id
         }
         state_set(signal_key, safe_summary)
+    if str_eq(conv_node_id, "") {
+        println("[chat] auto_persist: engram_node_full returned empty — conversation node lost (ts=" + ts_str + ")")
     }
 }