manager: add optional agent param to reminder RPCs

CountPendingReminders and ReminderRollup were hardcoded to
MANAGER_AGENT. Both now take agent: Option<String> — None keeps the
current behavior (manager's own), Some(name) returns that agent's
reminder stats. The broker functions already take an agent name, so
this is a thin wire-protocol change. Callers (web UI stats page,
post-turn counts) pass None.

Closes #122

hive-ag3nt/src/bin/hive-m1nd.rs

@@ -343,7 +343,7 @@ async fn fetch_manager_post_turn_counts(socket: &Path) -> (Option<u64>, Option<u
     };
     let reminders = match client::request::<_, ManagerResponse>(
         socket,
-        &ManagerRequest::CountPendingReminders,
+        &ManagerRequest::CountPendingReminders { agent: None },
     )
     .await
     {