Docs: Explain `GITHUB_TOKEN` forwarding for eval

[elgreco247]

Overview

Docker Agent does not automatically forward GITHUB_TOKEN or GH_TOKEN into eval containers, even when defined in the shell or ~/.config/cagent/.env. This is intentional because GitHub tokens are broad credentials, but the current docs do not make it clear.

Add a warning to the evaluation and GitHub Copilot provider docs:

Eval cases run in isolated containers. For security reasons, GITHUB_TOKEN and GH_TOKEN are not forwarded automatically. Explicitly pass the required token by name:

   docker agent eval agent.yaml ./evals -e GITHUB_TOKEN

When using a custom env file, both flags are required:

   docker agent eval agent.yaml ./evals \
     --env-from-file /path/to/secrets.env \
     -e GITHUB_TOKEN

Also clarify that host-side judge validation may succeed while the evaluated agent fails if the token is not forwarded.

Motivation

Env vars for other providers are forwarded to eval containers automatically and it is confusing that this doesn't happen for github-copilot.

Use cases

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Proposed solution

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Alternatives

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Related issues

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Additional context

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