add reminder rollup RPC and broker query

Surface reminder activity statistics (scheduled, delivered, pending counts)
for each agent over configurable time windows. Needed by the per-agent
stats page to display reminder metrics.

Adds:
- ReminderStats struct and ReminderRollup request/response variants
- Broker::reminder_rollup_for(agent, since_secs) method
- Agent and manager socket handlers for the new RPC
- SocketReply mapping for response conversion

hive-c0re/src/manager_server.rs

@@ -369,6 +369,14 @@ async fn dispatch(req: &ManagerRequest, coord: &Arc<Coordinator>) -> ManagerResp
                 },
             }
         }
+        ManagerRequest::ReminderRollup { since_secs } => {
+            match coord.broker.reminder_rollup_for(MANAGER_AGENT, *since_secs) {
+                Ok(stats) => ManagerResponse::ReminderRollup(stats),
+                Err(e) => ManagerResponse::Err {
+                    message: format!("{e:#}"),
+                },
+            }
+        }
         ManagerRequest::Whoami => ManagerResponse::Whoami {
             name: MANAGER_AGENT.to_owned(),
             role: "manager".to_owned(),