fix(reliability): state management

sessions.el

@@ -104,6 +104,8 @@ fn session_create(body: String) -> String {
     // Newest sessions first (prepend).
     // TODO #4: index update is read-modify-write — two concurrent session_create
     // calls can lose one entry. EL has no CAS primitive; fix requires runtime support.
+    // TODO(reliability #2): session_index RMW is non-atomic. Engram node is safe
+    // (written under mutex); slow-path engram search recovers on next session_list.
     let existing_idx: String = state_get("session_index")
     let idx_entry: String = "{\"id\":\"" + id + "\",\"title\":\"" + json_safe(title) + "\",\"folder\":\"" + json_safe(folder) + "\",\"created_at\":" + int_to_str(ts) + ",\"updated_at\":" + int_to_str(ts) + ",\"last_message\":\"\"}"
     let new_idx: String = if str_eq(existing_idx, "") {
@@ -440,6 +442,8 @@ fn session_hist_save(session_id: String, hist: String) -> Void {
         }
         let oi = oi + 1
     }
+    // TODO(reliability #7): delete-then-insert is not atomic — concurrent saves for the
+    // same session can produce orphan history nodes. State is primary truth; engram fallback.
     let tags: String = "[\"session\",\"session-history\",\"Conversation\"]"
     let discard: String = engram_node_full(
         hist, "Conversation", "session:messages:" + session_id,