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.

README.md

@@ -100,7 +100,7 @@ box it gets:
   at `/knowledge`, plus hyperhive's own docs in the container. →
   [knowledge](docs/integrations/knowledge.md)
 - **web UI** — a per-agent page behind the gateway: live terminal of the
-  turn stream, model / effort switching, cancel, `/compact`.
+  turn stream, model / effort switching, cancel.
 - **GUI** (opt-in, `services.hyperhive.agent.gui.enable`) — a Wayland
   desktop with screenshot / keyboard / mouse tools, viewable over VNC in
   the web UI.