Add environment validation script and npm validate:env command

[KGFCH2]

Summary

Add a utility script that validates the local .env file against .env.example and expose a convenient npm script for developers.

Details

• Adds scripts/validate-env.js
• Adds npm run validate:env
• Helps prevent missing or stale environment variables in local development

Acceptance Criteria

• .env.example and .env.local comparison runs cleanly
• npm run validate:env exits successfully when env vars match
• Useful error output is shown for missing or extra vars