drop the host side of the agent broker send

Remove the agent-side Send and AckUntil requests, their hive-c0re handlers
(handle_send with its 4 KiB size cap, handle_ack_until) and Broker::ack_until.
Everything that existed only for them goes with them: the OkWarn response
(its sole producer was Send's reserved-name warning), Recv's max parameter
and RECV_BATCH_MAX (the harness never asked for more than one message),
OPERATOR_SEND_REFUSAL, and the MESSAGE_MAX_BYTES / check_size limit (the
reminder timer keeps its own copy of the cap, so its comment is reworded).

Messages.remaining stays on the wire (default on read) because a harness
built before this change still requires it. Update the conventions docs.

hive-agent/src/main.rs

@@ -374,7 +374,6 @@ impl Surface for AgentSurface {
             socket,
             &Request::Recv {
                 wait_seconds: Some(RECV_WAIT.as_secs()),
-                max: None,
             },
             CONTROL_SOCKET_RETRY,
             RECV_RESPONSE_TIMEOUT,