gateway: converge, don't start/stop/reload

The gateway is a deployment (castle-gateway) — its lifecycle is the same
convergence as everything else, so the bespoke start/stop/reload verbs
(which still routed through the retired enable/disable path) are gone.

- CLI: `castle gateway` is now inspection only — bare or `status` shows
  the route table. Start/stop/reload it via `castle apply` (render routes
  + reload) or `castle restart castle-gateway`. Deletes the last users of
  `_service_enable`/`_service_disable`, so those are removed too.
- API: retire `POST /gateway/reload` (making routes live is `POST /apply`);
  `PUT /gateway/config` message + docstring now say apply, not deploy.
- UI: GatewayPanel's "Reload" button → "Apply" (useApply); drop the
  useGatewayReload hook; GatewaySettings points at `castle apply`.
- docs: `castle gateway reload|status` → `castle gateway` (status lens).

core 129 / cli 31 / api 59 pass; dashboard builds clean; live `castle
gateway` shows routes with no start/stop/reload; /gateway/reload removed
from the API.

README.md

@@ -182,7 +182,7 @@ castle list [--kind K] [--stack S] [--json]   # catalog + every deployment view
 castle status                                 # unified runtime status
 castle doctor                                 # diagnose setup + health, with fix hints
 castle restart [name]                         # imperative bounce (one or all)
-castle gateway reload|status
+castle gateway               # status + route table (inspection)
 ```
 
 To turn a deployment off, set `enabled: false` in its config and `castle apply` —