docs: merge network-model stub into network hub, improve bridge deprecation

- network.md: add Core model prose (loopback-first, canvas host, remote access)
  from the 22-line network-model.md stub
- network-model.md: add redirect note pointing to /network#core-model
- bridge-protocol.md: replace scattered deprecation notes with prominent
  <Warning> callout at the top

docs/gateway/network-model.md

@@ -7,6 +7,8 @@ title: "Network model"
 
 # Network Model
 
+> This content has been merged into [Network](/network#core-model). See that page for the current guide.
+
 Most operations flow through the Gateway (`openclaw gateway`), a single long-running
 process that owns channel connections and the WebSocket control plane.