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/src/turn.rs

@@ -201,8 +201,8 @@ pub enum TurnError {
     /// provider (e.g. HTTP 429) without reporting it ends up here. Handled
     /// like [`Self::ApiStall`].
     AgentStall(String),
-    /// A hard failure with no recovery — the serve loop escalates it to the
-    /// operator (`send_to_operator`).
+    /// A hard failure with no recovery. Logged as `turn failed`, the exact
+    /// message the `hh-agent-turn-failed` Grafana alert matches.
     Failed(anyhow::Error),
 }
 
@@ -604,6 +604,8 @@ pub fn emit_turn_end(bus: &Bus, outcome: &TurnOutcome) {
                 ok: false,
                 note: Some(note.clone()),
             });
+            // The `hh-agent-turn-failed` alert in swarm-grafana.nix matches
+            // this message exactly.
             tracing::warn!(error = %note, "turn failed");
         }
     }