Document all supported environment variables in a single reference page

[himanshu231204]

Description

OpenAgent Eval supports configuration via environment variables (API keys, timeouts, etc.), but there's no single reference listing all supported environment variables and what they do. Users have to dig through source code to discover them.

What to Do

1. Search the codebase for \os.getenv, \os.environ, and \env_var\ patterns to find all supported environment variables
2. Document them in a new page (\docs/environment-variables.md) or a dedicated section
3. Include: env var name, what it controls, default value, and example

Files to Look At

• All files under \openagent_eval/\ — search for env variable usage
• \docs/installation.md\ or \docs/cli.md\ — potential location for the reference

Good First Issue ✅

A research + documentation task that doesn't require writing code.