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-ag3nt/src/mcp.rs

@@ -48,6 +48,7 @@ pub enum SocketReply {
     Logs(String),
     LooseEnds(Vec<hive_sh4re::LooseEnd>),
     PendingRemindersCount(u64),
+    ReminderRollup(hive_sh4re::ReminderStats),
     Whoami {
         name: String,
         role: String,
@@ -68,6 +69,7 @@ impl From<hive_sh4re::AgentResponse> for SocketReply {
             hive_sh4re::AgentResponse::PendingRemindersCount { count } => {
                 Self::PendingRemindersCount(count)
             }
+            hive_sh4re::AgentResponse::ReminderRollup(stats) => Self::ReminderRollup(stats),
             hive_sh4re::AgentResponse::Whoami {
                 name,
                 role,
@@ -95,6 +97,7 @@ impl From<hive_sh4re::ManagerResponse> for SocketReply {
             hive_sh4re::ManagerResponse::PendingRemindersCount { count } => {
                 Self::PendingRemindersCount(count)
             }
+            hive_sh4re::ManagerResponse::ReminderRollup(stats) => Self::ReminderRollup(stats),
             hive_sh4re::ManagerResponse::Whoami {
                 name,
                 role,