fix: revert openshell.md — native-sandbox stays on separate Dockerfile

native-sandbox is not included in the unified Dockerfile migration for now.
Keep the original openab-native-sandbox image reference.

Author: chaodu-agent

docs/openshell.md

@@ -75,7 +75,7 @@ Using the pre-built image:
 
 ```bash
 openshell sandbox create --name oab \
-  --from ghcr.io/openabdev/openab:beta-native-sandbox \
+  --from ghcr.io/openabdev/openab-native-sandbox:latest \
   --provider discord \
   -- bash
 ```