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/agent_server.rs

@@ -207,6 +207,14 @@ async fn dispatch(req: &AgentRequest, agent: &str, coord: &Arc<Coordinator>) ->
                 },
             }
         }
+        AgentRequest::ReminderRollup { since_secs } => {
+            match coord.broker.reminder_rollup_for(agent, *since_secs) {
+                Ok(stats) => AgentResponse::ReminderRollup(stats),
+                Err(e) => AgentResponse::Err {
+                    message: format!("{e:#}"),
+                },
+            }
+        }
         AgentRequest::Whoami => AgentResponse::Whoami {
             name: agent.to_owned(),
             role: "agent".to_owned(),