Document all backend environment variables in .env.example (#3425)
* Initial plan
* Document all backend environment variables in .env.example
Co-authored-by: bodhish <14979190+bodhish@users.noreply.github.com>
* cleanup unused variables
* remove hcx from default prebuilt env
* remove config variable causing duplicate name identifiers
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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bodhish <14979190+bodhish@users.noreply.github.com>
Co-authored-by: Aakash Singh <mail@singhaakash.dev>
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