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.
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