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,

hive-agent/src/reminder_timer.rs

@@ -33,11 +33,7 @@ const REMINDER_BATCH_LIMIT: u64 = 100;
 /// Poll interval — matches the old c0re `POLL_INTERVAL`.
 const POLL_INTERVAL: Duration = Duration::from_secs(5);
 
-/// Same cap the broker enforces on `send`/`remind` bodies
-/// (`hive-c0re`'s `agent_config::limits::MESSAGE_MAX_BYTES`, not
-/// reachable from here — hive-agent doesn't depend on hive-c0re).
-/// Duplicated rather than shared: this is the last remaining reminder
-/// caller of that constant once the c0re-side store is deleted (commit 6).
+/// Cap on the inline form of a `remind` body; a longer body goes to a file.
 const REMINDER_BODY_MAX_BYTES: usize = 4096;
 
 /// Maximum pending (undelivered) reminders this agent may hold at once.