Rename image-registry to registry-credentials to avoid collision

The existing 'image-registry' key is used for pushing images to a remote
registry (URL string). Rename the new auth config to 'registry-credentials'
to avoid collision.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

stack_orchestrator/deploy/spec.py

@@ -103,8 +103,11 @@ class Spec:
 
         Used for private container registries like GHCR. The token-env field
         specifies an environment variable containing the API token/PAT.
+
+        Note: Uses 'registry-credentials' key to avoid collision with
+        'image-registry' key which is for pushing images.
         """
-        return self.obj.get("image-registry")
+        return self.obj.get("registry-credentials")
 
     def get_volumes(self):
         return self.obj.get(constants.volumes_key, {})