agents: remove send(operator) and the operator inbox; alert on turn failures

Agents reach the operator over matrix now (#5115). A `send` to
`operator` is refused, by the broker and by the MCP allow-list check,
with an error that names the matrix MCP tools. The dashboard's Y3R C4LL
tab goes with it: the operator inbox, `GET /api/operator-inbox`,
`Broker::unread_for_recipient`, and the browser notifications that only
ever fired on operator-bound messages.

The harness no longer posts turn failures and plugin-install failures
to the operator inbox. Both are already logged at warn; a new Grafana
rule, `hh-agent-turn-failed`, fires per agent on the harness's
`turn failed` line.

Dashboard-to-agent messaging (op-send, the agent web UI's message box,
mark-all-read for agent inboxes) is unchanged.

Decisions per mara on #5115: q1 a, q2 a plus a grafana alert, q3 keep
dashboard-to-agent messaging and leave swarm-level code alone.

hive-agent-mcp/src/mcp/args.rs

@@ -7,7 +7,7 @@ use rmcp::schemars;
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct SendArgs {
     /// Logical agent name to deliver the message to (e.g. `"manager"`,
-    /// `"alice"`, or the literal `"operator"` for the dashboard's T4LK box).
+    /// `"alice"`). `"operator"` is refused: reach the operator over matrix.
     pub to: String,
     /// Message body. Plain text; the broker doesn't parse it.
     pub body: String,