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.

nix/agent-modules/mcp.nix

@@ -51,14 +51,13 @@ in
     description = ''
       Names this agent is allowed to `send` to via
       `mcp__hyperhive__send`. Empty list (the default) means
-      unrestricted — the agent can message any peer, the
-      operator, or the manager. Non-empty list constrains the
+      unrestricted — the agent can message any peer or the
+      manager. Non-empty list constrains the
       surface: only the listed names + the manager (always
       allowed) get through; anything else returns an error
-      string to claude without touching the broker. The
-      operator (`operator`) needs to be in the list if the
-      agent should be able to surface output on the
-      dashboard.
+      string to claude without touching the broker.
+      `operator` is refused either way: agents reach the operator
+      over matrix.
 
       Useful for sandboxing untrusted sub-agents — set
       `[ "manager" ]` to scope them to manager-only chatter.