chore(gateway): exclude config.json from repo, add example config

config.json contains deployment-specific values and should not be versioned.
config.example.json serves as a template for new deployments.

.gitignore

@@ -2,6 +2,7 @@
 .zed
 .vscode
 .idea
+HEAD
 
 # Local
 .claude/
@@ -10,3 +11,4 @@
 # Secrets
 **/secrets/
 **/.env
+gateway/config.json