[jsulopzs]

Problem

Since #247, authenticate with chat-gpt or chat-gpt-device-code succeeds at once when a working ChatGPT login exists. That is the right default, but it leaves a client no way to switch to another ChatGPT account: the only path is logout first, which leaves the user without any login if the new sign-in is cancelled or fails.

Fix

• Both ChatGPT methods advertise _meta: { replaceLogin: true } in authMethods, on ACP v1 and v2.
• When authenticate carries _meta: { replaceLogin: true }, the adapter skips the working-login check and starts the login (account/login/start) as usual: the browser flow for chat-gpt, the URL elicitation for chat-gpt-device-code.
• Codex keeps the current account until the new login completes, and replaces it only then. A cancelled or failed login leaves the current account untouched.
• Without _meta.replaceLogin, nothing changes.

Verification

• Tests: the replacement browser login starts with a working login and keeps the current account until completion (success and failure); the device-code replacement starts and keeps the account when cancelled; v1 and v2 initialize advertise replaceLogin on both methods. The new tests fail without the change. Build, typecheck and the full suite pass.
• Through a real client (CodeCompanion over ACP) with a working ChatGPT login: the replacement chat-gpt login opened the browser sign-in page; closing it without signing in left the same account in place and the session kept working.

This touches the same functions as #696 (login cancellation and isolation); whichever lands second needs a small rebase.