chore: improve .env.example readability

[JonesRoy3]

This PR reformats .env.example to make it easier to use:

• Each environment variable is now on its own line
• Added brief comments explaining how to copy and use the file
• Emphasizes using test/dev keys when experimenting locally This improves readability and aligns better with typical dotenv tooling. No runtime behavior is changed.

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[!NOTE] Add header and usage comments to .env.example to improve readability; no code/runtime changes.

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