fix(docs): avoid printing GITHUB_TOKEN in cloud quickcheck

[chaliy]

Motivation

• Prevent accidental disclosure of GITHUB_TOKEN in cloud-agent transcripts and logs by replacing a quickcheck that printed secret values.

Description

• Update AGENTS.md quickcheck to use a presence-only test: replace doppler run -- env | rg 'GITHUB_TOKEN' with doppler run -- bash -lc 'test -n "${GITHUB_TOKEN:-}" && echo GITHUB_TOKEN present', keeping the existing gh auth status check unchanged.

Testing

• Documentation-only change; no automated tests were modified or required locally, and the repository CI is expected to run the standard test suite on the branch.

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Codex Task