Hidden gateway JSON sign-in mode for claude auth login

What

claude auth login is the command that signs Claude Code in. It gains a hidden option, --gateway-json, that does not show in the help. Instead of talking to a person, it writes the Cloud gateway sign-in questions as JSON lines (one machine-readable message per line) and reads the answers from standard input, so another program can show the sign-in to the person.

• It is rejected when combined with --console, --claudeai, --sso or --email.
• When a gateway is configured, it refuses to run in a terminal a person is typing into, saying the command is run by the VS Code extension.
• It refuses to run inside a Claude Code session.
• If no gateway is available, it prints an error saying sign-in "isn't available with this machine's settings". What makes a gateway available was not traced.
• An interactive sign-in screen for the gateway case is also added for use in a terminal.
• claude auth status, when asked for JSON output, now reports gatewaySignInOffered when that same gateway check finds one.
• A new rule matches auth login when gatewayJson is on (or a helper returns true) and adds it to an existing list of exempted commands. What that list controls was not read.

Before, claude auth login only handled email, SSO, Console and claude.ai sign-in, and a gateway setup just printed a note to run the interactive /login.

Why

This gives editor extensions a way to drive gateway sign-in themselves rather than sending you to /login. Because the option is hidden, it is meant for those programs rather than for typing by hand.