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/dashboard.css

@@ -475,6 +475,63 @@ summary:hover { color: var(--purple); }
 .msg-sep { color: var(--muted); }
 .msg-to { color: var(--pink); }
 .msg-body { color: var(--fg); white-space: pre-wrap; word-break: break-word; }
+.op-compose {
+  position: relative;
+  display: flex;
+  align-items: flex-start;
+  gap: 0.6em;
+  margin-top: 0.4em;
+  padding: 0.55em 0.8em;
+  background: rgba(24, 24, 37, 0.85);
+  border: 1px solid var(--border);
+  border-top: none;
+}
+.op-compose-prompt {
+  color: var(--purple);
+  text-shadow: 0 0 4px currentColor;
+  font-weight: bold;
+  white-space: nowrap;
+  user-select: none;
+  padding-top: 0.15em;
+}
+.op-compose-input {
+  flex: 1;
+  background: transparent;
+  border: none;
+  outline: none;
+  color: var(--fg);
+  font: inherit;
+  font-size: 0.85em;
+  line-height: 1.5;
+  resize: none;
+  overflow: hidden;
+  min-height: 1.5em;
+  caret-color: var(--purple);
+}
+.op-compose-input::placeholder { color: var(--muted); }
+.op-compose-suggest {
+  position: absolute;
+  bottom: 100%;
+  left: 0.8em;
+  margin-bottom: 0.2em;
+  background: rgba(24, 24, 37, 0.95);
+  border: 1px solid var(--border);
+  font-size: 0.85em;
+  min-width: 12em;
+  max-height: 12em;
+  overflow-y: auto;
+  z-index: 10;
+}
+.op-compose-suggest .item {
+  padding: 0.2em 0.8em;
+  cursor: pointer;
+  color: var(--fg);
+}
+.op-compose-suggest .item.active,
+.op-compose-suggest .item:hover {
+  background: rgba(203, 166, 247, 0.18);
+  color: var(--purple);
+}
 footer {
   margin-top: 4em;
   text-align: center;