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.

docs/web-ui/README.md

@@ -75,8 +75,8 @@ JS renders — no full-page reloads.
 - **[Dashboard layout](dashboard.md)** — every tab and standalone page,
   in full implementation detail: endpoint shapes, event wiring, exact
   badge-derivation rules.
-- **[Per-agent page](agent.md)** — the per-agent terminal, composer,
-  side panel, slash commands, and per-agent endpoints.
+- **[Per-agent page](agent.md)** — the per-agent terminal,
+  side panel, actions, and per-agent endpoints.
 - **[Shape (shared by both)](shape.md)** — the SPA skeleton, SSE
   multiplexing, terminal pane, listener bind, per-agent relative
   paths, `data-async` form pattern, atomic repaint.
@@ -106,8 +106,8 @@ touching the code rather than using the UI:
   [`terminal-rendering.md`](terminal-rendering.md) (full row taxonomy
   and dispatch walkthrough); for a high-level summary see
   [`agent.md`](agent.md) (Per-stream rendering).
-- **"What slash commands does the agent accept?"** →
-  [`agent.md`](agent.md) (Terminal-embedded prompt).
+- **"What actions does the agent page offer?"** →
+  [`agent.md`](agent.md) (Actions).
 - **"What are the per-agent HTTP endpoints?"** →
   [`agent.md`](agent.md) (Per-agent endpoints).
 - **"Which CSS variable to use / where are colours defined?"** →