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.

docs/developing-castle.md

@@ -51,7 +51,7 @@ for tools/libraries.
 - Catalog + editing: `GET /programs[/{name}]`, `POST /programs/{name}/{action}`,
   `PUT|DELETE /programs/{name}`; likewise `/services`, `/jobs`
 - Config: `GET|PUT /`, `POST /apply`, `POST /deploy`
-- Gateway: `GET /gateway`, `GET /gateway/caddyfile`, `POST /gateway/reload`
+- Gateway: `GET /gateway`, `GET /gateway/caddyfile` (reload is convergence: `POST /apply`)
 - Mesh: `GET /mesh/status`, `GET /nodes[/{hostname}]`
 - Agents (dashboard terminal UX): `GET /agents`, `GET /agents/sessions`,
   `GET /agents/history`, `DELETE /agents/sessions/{id}`, `WS /agents/{name}/session`