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.

hive-core-agent-sock/src/lib.rs

@@ -44,11 +44,6 @@ pub enum Request {
     /// Non-mutating: how many pending messages are addressed to me?
     /// Used by the harness to render a status line after each tool call.
     Status,
-    /// Operator-injected message TO this agent (from this agent's own web
-    /// UI). Recipient is implicit — `from` is `"operator"`. Effectively the
-    /// per-agent equivalent of the old dashboard T4LK form, but scoped to
-    /// the agent whose page the operator is on.
-    OperatorMsg { body: String },
     /// Wake-up event injected from inside the container. Recipient is
     /// implicit (this agent); `from` is caller-chosen. See
     /// `docs/process/conventions.md::Wake injection` for the trust model and