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/agent.css

@@ -150,6 +150,12 @@ pre.diff {
 .agent-inbox .inbox-from { color: var(--amber); }
 .agent-inbox .inbox-sep  { color: var(--muted); }
 .agent-inbox .inbox-body { color: var(--fg); white-space: pre-wrap; word-break: break-word; }
+.agent-inbox li.inbox-reply {
+  padding-left: 1em;
+  border-left: 2px solid var(--border);
+  margin-left: 0.4em;
+}
+.agent-inbox .inbox-reply-tag { color: var(--muted); font-size: 0.85em; }
 
 .agent-inbox .answer-form {
   grid-column: 1 / -1;

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), ' ',