Gateway restart notices: explicit status{restarting} signal, correct timing

The app previously showed 'Gateway restarting' on every connection loss.
Now the gateway broadcasts status{state=restarting} on its shutdown path
(before closing the sockets), and the app:

- posts 'Gateway restarting' immediately on that frame (not on the
  socket-drop transition, which lags by the ~20s WS ping timeout)
- posts 'Gateway online' on the next reconnect only when the restart
  notice was posted (latch) - a plain network drop shows neither, just
  the reconnect banner
- drops the 'Gateway is restarting...' banner (replaced by the chat notice)

Docs (04-wire-protocol, frames.schema.json) updated: restarting is no
longer reserved. Test for the disconnect broadcast added to the local
hermes-agent test mirror (git-ignored, not committed).

docs/protocol/frames.schema.json

@@ -59,7 +59,7 @@
       "search.results": { "payload": { "query": { "type": "string" }, "scope": { "type": "string", "enum": ["all", "chat"] }, "hits": { "type": "array", "items": { "type": "object", "properties": { "message_id": {"type":"string"}, "chat_id": {"type":"string"}, "thread_id": {"type":["string","null"]}, "role": {"type":"string"}, "snippet": {"type":"string"}, "ts": {"type":"integer"} } } } } },
       "media.offer": { "description": "Agent-sent media available; app pulls bytes.", "payload": { "$ref": "#/definitions/media_ref" } },
       "read.receipt": { "description": "Agent received and started processing the user's message; app shows ✓✓ on user bubbles. Emitted to the originating connection when a message.send is accepted for processing.", "payload": { "message_id": { "type": "string" } } },
-      "status": { "description": "Gateway health state; broadcast to all connected clients at startup (state=online).", "payload": { "state": { "type": "string", "enum": ["online", "restarting", "degraded"] } } },
+      "status": { "description": "Gateway health state; broadcast to all connected clients at startup (state=online) and to late joiners on hello.ack. state=restarting is broadcast on the gateway's shutdown path (restart/stop) before the sockets close; the app shows the 'Gateway restarting' chat notice only on that signal, not on a plain network drop.", "payload": { "state": { "type": "string", "enum": ["online", "restarting", "degraded"] } } },
       "error": { "payload": { "code": { "type": "string", "enum": ["auth", "not_found", "rate_limited", "media_too_large", "unsupported", "internal"] }, "message": { "type": "string" } } },
       "pong": { "payload": { "ts": { "type": "integer" } } },
       "sync.done": { "payload": { "cursor": { "type": "integer" } } },