render message reply threads in dashboard and per-agent inbox

- MessageEvent and DashboardEvent Sent/Delivered now carry id and in_reply_to
- broker.send() includes last_insert_rowid in the emitted event
- recent_all() and recv_batch() include id and in_reply_to from the DB
- deliver_reminders_batch() tracks per-row rowids within the transaction
- dashboard message flow: reply rows are indented with a border-left and a
  clickable '↳ reply' tag that scroll-jumps + briefly highlights the parent
- per-agent inbox: reply messages get a '↳ reply ·' prefix and indent

Closes #26

hive-c0re/src/main.rs

@@ -257,25 +257,29 @@ fn spawn_broker_to_dashboard_forwarder(coord: Arc<Coordinator>) {
     tokio::spawn(async move {
         loop {
             match rx.recv().await {
-                Ok(MessageEvent::Sent { from, to, body, at }) => {
+                Ok(MessageEvent::Sent { id, from, to, body, at, in_reply_to }) => {
                     let file_refs = dashboard::scan_validated_paths(&body);
                     coord.emit_dashboard_event(DashboardEvent::Sent {
                         seq: coord.next_seq(),
+                        id,
                         from,
                         to,
                         body,
                         at,
+                        in_reply_to,
                         file_refs,
                     });
                 }
-                Ok(MessageEvent::Delivered { from, to, body, at }) => {
+                Ok(MessageEvent::Delivered { id, from, to, body, at, in_reply_to }) => {
                     let file_refs = dashboard::scan_validated_paths(&body);
                     coord.emit_dashboard_event(DashboardEvent::Delivered {
                         seq: coord.next_seq(),
+                        id,
                         from,
                         to,
                         body,
                         at,
+                        in_reply_to,
                         file_refs,
                     });
                 }