hive-agent: the per-agent page has no message box; its send route and the broker's operator message are removed

The page's composer (its input, slash commands and completion), POST /send in
hive-agent, the OperatorMsg request of the agent socket and its handler in
hive-c0re, and the broker-error helper only the send route used are deleted:
writing to an agent is the swarm UI's terminal. cancel turn, new session and
log out stay in the state badge's menu; POST /api/compact stays, with nothing
on the page calling it. The request-line limit keeps its size, and its comment
says it is a generous bound now. The docs describe the actions in place of the
footer and the prompt.

frontend/packages/agent/src/index.html

@@ -18,7 +18,7 @@
   </head>
   <body class="agent-shell">
     <!-- The Preact rewrite owns everything below — header, terminal,
-       composer, side panel. See main.tsx/Root.tsx for the component
+       side panel. See main.tsx/Root.tsx for the component
        tree; docs/web-ui/agent.md for the design. -->
     <div id="preact-root"></div>