A deletion instruction consumed by LangEx.Message.add_messages/2.
Emitted (never stored) so a reducer update can prune history instead of
only appending. id targets a single message by its :id; the special
LangEx.Message.remove_all/0 sentinel clears every prior message,
letting a node replace the whole history (e.g. summarization).
Summary
Types
@type t() :: %LangEx.Message.RemoveMessage{id: String.t()}