remove the needs_update system event: nothing emits it

Refs #5948

nix/reserved-names.nix

@@ -34,8 +34,9 @@
   # (`{from: "operator", ...}`) and the fallback attribution for an answered
   # question.
   "operator"
-  # Helper events (`container_crash`, `needs_update`) — the sender an
-  # agent is told to treat as hyperhive itself rather than as a peer.
+  # The sender of plain-text notices from hive-c0re (knowledge updates,
+  # the graceful-stop wake) — an agent is told to treat it as hyperhive
+  # itself rather than as a peer.
   # `hive_sh4re::manager::SYSTEM_SENDER`.
   "system"
   # A due self-scheduled reminder arrives as its own sender, so a wake I asked