Built-in gh api stand-in for sessions without the GitHub CLI

What

gh is GitHub's command-line tool. When a session finds no GitHub CLI at startup, Claude Code now provides its own stand-in:

• What it does: it handles only gh api <REST endpoint>, sending the request through the session's GitHub proxy without a token.
• Supported options: --method, --field, --raw-field, --header, --input, --paginate, --jq, --include and --hostname.
• Placeholders: {owner}, {repo} and {branch} are filled from GH_REPO or the git remotes.
• Refusals: it refuses GraphQL, unsupported options and headers that are not on its allowed list.
• Off switch: it is disabled when CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC is set.
• How it runs: it is a per-session script that calls Claude Code with a hidden --gh-standin argument, and a new ghStandinMain entry handles it.
• Which gh wins: when the stand-in is in use and a PATH is set, its folder is added after the PATH instead of before it, so a real gh runs if one is there.
• Claude is told: the instructions about the session proxy now say no GitHub CLI was found and that gh here is a built-in stand-in that supports only gh api <REST endpoint>.

The --daemon-worker, remote-control and agents startup paths also now call a setup step for global agents, and a new startup check was added at entry.

Why

Sessions without gh installed can still make GitHub REST calls. Other gh commands will not work there, and Claude is told so.