dashboard: terminal compose with @-mention sticky recipient

new section under MESS4GE FL0W. msgflow already tails only
broker traffic (sent + delivered), which is exactly the
'messages through core' view the operator wants; no
per-agent thinking leaks through. compose box below:

- a prompt span renders the sticky recipient ('@coder>'),
  rendered outside the textarea so it can't be edited
  inadvertently. on submit the recipient gets persisted to
  localStorage so it survives reload.
- start the input with '@name body' to redirect — the parser
  splits at the first whitespace and the new recipient
  becomes sticky.
- typing '@' at the start opens a completion dropdown over
  the textarea pulled from window.__hyperhive_state.containers;
  arrow keys cycle, tab/enter selects, escape closes. clicking
  works too.
- manager swap: agents flagged is_manager are surfaced as
  '@manager' (the broker's recipient string) instead of
  '@hm1nd' (the container name), so the message actually
  routes to the manager's inbox.

backend: new POST /op-send accepts {to, body} and drops a
broker.send({from:'operator', to, body}) — same shape as the
per-agent web UI's OperatorMsg, but lets the operator choose
the recipient explicitly from the main dashboard.

hive-c0re/assets/index.html

@@ -49,8 +49,15 @@
 
   <h2>◆ MESS4GE FL0W ◆</h2>
   <div class="divider">══════════════════════════════════════════════════════════════</div>
-  <p class="meta">live tail — newest at the top. tap on every <code>send</code> / <code>recv</code> through the broker.</p>
+  <p class="meta">live tail — newest at the top. tap on every <code>send</code> / <code>recv</code> through the broker. compose below: <code>@name</code> picks the recipient (sticky until you @ someone else); <code>tab</code> completes.</p>
   <div id="msgflow" class="msgflow"><span class="meta">connecting…</span></div>
+  <div id="op-compose" class="op-compose">
+    <span id="op-compose-prompt" class="op-compose-prompt">@—&gt;</span>
+    <textarea id="op-compose-input" class="op-compose-input"
+              placeholder="@agent message…  (enter sends, shift+enter newline, tab completes @-mention)"
+              rows="1" autocomplete="off"></textarea>
+    <div id="op-compose-suggest" class="op-compose-suggest" hidden></div>
+  </div>
 
   <footer>
     <div class="divider">══════════════════════════════════════════════════════════════</div>