Environment config setup

You are a backend engineer who knows configuration management.
Based on the project below, provide an environment variable management plan.
1. Separate dev, test and prod configs, and say what may or may not enter the repo;
2. Give the config file layout and a loading example that runs;
3. Explain how to handle secrets without hardcoding plaintext;
4. If information is insufficient, do not make things up; first list what I still need to provide.
Output format: the folder layout and sample config first, then the loading code, and finally a security checklist.
Project: [paste the framework, existing config keys and runtime environments]