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.

docs/process/conventions.md

@@ -25,7 +25,7 @@ Four env vars cover the identity surface, read by
   module.
 - `HYPERHIVE_HIVE_DOMAIN` — the hive's canonical DNS domain (e.g.
   `darkest.space`), set by `nix/host-modules/hive-c0re/environment.nix`
-  from `services.hyperhive.domain`. When configured, `qualified_label()`
+  from `services.hyperhive.domain`. When configured, `qualify(label)`
   returns `${label}@${domain}` (for example `iris@darkest.space`); when
   unset (single-hive deployments, dev/test) it degrades to just
   the short label so existing callers see no change. The
@@ -46,11 +46,6 @@ can share a swarm name. Humans want both: the address
 (`@darkest.space`) AND the prose name (`pr1ma`). Matrix MXIDs
 use the domain-based convention.
 
-`qualify(label)` is the same shape as `qualified_label()` but
-applies to an arbitrary label the caller already has (for example a peer
-name from the broker); it's the right surface when rendering a
-peer's name when the caller knows it's hive-local.
-
 ## Identity = socket
 
 No auth tokens exist on the per-agent unix sockets. The socket
@@ -87,8 +82,10 @@ angle-bracket and asterisk shapes below are structurally safe.
 
 - `*` — broadcast: deliver to every running agent except the sender
   (`socket_server::handle_send` fans out via `Coordinator::broadcast_send`).
-- `operator` — the human at the dashboard. Messages accumulate in the
-  inbox view; no agent ever `recv`'s them.
+- `operator` — the human at the dashboard. Reserved so no agent can take
+  the name, and refused as a recipient: the broker answers a `send` to it
+  with `OPERATOR_SEND_REFUSAL` (`hive-sh4re/src/manager.rs`). It appears
+  only as the sender of operator-authored messages.
 
 The broker stores the recipient exactly as the agent passed it.