add .env.example with sample environment variables

[ishaanxgupta]

Solves issue #14988 Summary This PR adds a .env.example file to document all required environment variables for the project. It improves onboarding for new contributors and reduces confusion during local setup by providing a clear template they can copy into their own .env file.

Details Added a new .env.example file in the project root. Included placeholder environment variables based on what the CLI currently expects. No functional changes to the codebase—this is strictly a developer-experience improvement.