Genuine Claude Code CLI sub-requests (e.g. the security monitor) carry
no identity system prompt, so claude_code_only groups wrongly rejected
them with "this group only allows Claude Code clients". Detect the
x-anthropic-billing-header block with cc_entrypoint=cli as a stable
client signal, while keeping the existing header/metadata checks.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
