remove the needs_update system event: nothing emits it

Refs #5948

hive-c0re/src/coordinator.rs

@@ -936,10 +936,9 @@ impl Coordinator {
 }
 
 /// `push_todo` summary text for a rebuild outcome, for
-/// `job_queue::exec::run_emit_rebuilt`. Pure + independently testable, unlike
-/// the old `HelperEvent::Rebuilt`'s separate `sha`/`tag`
-/// fields — those become part of the summary text itself now, since a todo
-/// carries one string, not a structured payload.
+/// `job_queue::exec::run_emit_rebuilt`. Pure + independently testable; the
+/// `sha`/`tag` fields are part of the summary text, since a todo carries one
+/// string, not a structured payload.
 #[must_use]
 pub fn rebuilt_todo_summary(
     agent: &str,