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-ag3nt/assets/app.js

@@ -532,7 +532,10 @@
     list.innerHTML = '';
     const fmt = (n) => new Date(n * 1000).toISOString().replace('T', ' ').slice(5, 19);
     for (const m of rows) {
-      const li = el('li');
+      const li = el('li', m.in_reply_to != null ? { class: 'inbox-reply' } : {});
+      if (m.in_reply_to != null) {
+        li.append(el('span', { class: 'inbox-reply-tag' }, '↳ reply · '));
+      }
       li.append(
         el('span', { class: 'inbox-ts' }, fmt(m.at)), ' ',
         el('span', { class: 'inbox-from' }, m.from), ' ',