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/web_ui.rs

@@ -529,6 +529,8 @@ async fn fetch_reminder_stats(socket: &std::path::Path, flavor: Flavor, window_s
                 socket,
                 &hive_sh4re::ManagerRequest::ReminderRollup {
                     since_secs: window_secs,
+                    // Manager's own stats page — its own reminders.
+                    agent: None,
                 },
             )
             .await