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/assets/app.js

@@ -1631,8 +1631,16 @@
       if (bannerOffTimer) clearTimeout(bannerOffTimer);
       bannerOffTimer = setTimeout(() => banner.classList.remove('active'), 4000);
     }
+    // Map of broker row id → rendered row element. Lets reply rows add
+    // a visual "↳ in reply to" indicator that links back to the parent.
+    // Bounded by the history window (~200 msgs from /dashboard/history),
+    // well within normal memory.
+    const msgRowMap = new Map();
+
     function renderMsg(ev, api, glyph) {
-      const row = api.row('msgrow ' + ev.kind, '');
+      const isReply = ev.in_reply_to != null;
+      const cls = 'msgrow ' + ev.kind + (isReply ? ' msg-reply' : '');
+      const row = api.row(cls, '');
       // Build via DOM so path anchors stay live + escape rules are
       // automatic (text nodes don't need esc()).
       const ts = document.createElement('span');
@@ -1648,7 +1656,35 @@
       const body = document.createElement('span');
       body.className = 'msg-body';
       appendLinkified(body, ev.body, ev.file_refs);
-      row.append(ts, ' ', arrow, ' ', from, ' ', sep, ' ', to, ' ', body);
+      // Reply thread indicator: a small "↳ reply to <from>" hint that
+      // shows which message this is responding to. If we have the parent
+      // in our row map, clicking scrolls it into view.
+      if (isReply) {
+        const replyTag = document.createElement('span');
+        replyTag.className = 'msg-reply-tag';
+        const parentRow = msgRowMap.get(ev.in_reply_to);
+        if (parentRow) {
+          const link = document.createElement('a');
+          link.href = '#';
+          link.textContent = '↳ reply';
+          link.title = 'scroll to parent message';
+          link.addEventListener('click', (e) => {
+            e.preventDefault();
+            parentRow.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
+            parentRow.classList.add('msg-highlight');
+            setTimeout(() => parentRow.classList.remove('msg-highlight'), 1500);
+          });
+          replyTag.append(link);
+        } else {
+          replyTag.textContent = '↳ reply';
+        }
+        row.prepend(replyTag);
+        row.append(ts, ' ', arrow, ' ', from, ' ', sep, ' ', to, ' ', body);
+      } else {
+        row.append(ts, ' ', arrow, ' ', from, ' ', sep, ' ', to, ' ', body);
+      }
+      // Register this row so future replies can reference it.
+      if (ev.id != null && ev.id > 0) msgRowMap.set(ev.id, row);
     }
     HiveTerminal.create({
       logEl: flow,

hive-c0re/assets/dashboard.css

@@ -726,6 +726,30 @@ summary:hover { color: var(--purple); }
 }
 .live .msgrow.sent .msg-arrow { color: var(--cyan); }
 .live .msgrow.delivered .msg-arrow { color: var(--green); }
+/* Reply-thread rendering: indented border-left + muted reply tag. */
+.live .msgrow.msg-reply {
+  padding-left: 1.2em;
+  border-left: 2px solid var(--border);
+  margin-left: 0.6em;
+}
+.msg-reply-tag {
+  color: var(--muted);
+  font-size: 0.8em;
+  white-space: nowrap;
+  order: -1; /* prepend before other flex items */
+}
+.msg-reply-tag a {
+  color: var(--muted);
+  text-shadow: none;
+  font-weight: normal;
+}
+.msg-reply-tag a:hover { color: var(--fg); }
+/* Flash highlight when scrolled to from a reply link. */
+@keyframes msg-highlight-fade {
+  from { background: rgba(203, 166, 247, 0.18); }
+  to   { background: transparent; }
+}
+.msg-highlight { animation: msg-highlight-fade 1.5s ease-out forwards; }
 .msg-ts { color: var(--muted); font-size: 0.85em; }
 .msg-arrow { font-weight: bold; }
 .msg-from { color: var(--amber); }