Signing in can keep the stored login and warns when the previous one may remain

What

Signing in and switching accounts changed in two ways:

• keepStoredLogin: a new option for the reset that runs when you change account. When it is set, one cleanup step is skipped, so signing in again to the same organisation can keep the login that is already stored.
• departingLoginMayRemain: sign-in now notes when the previous login may still be stored. In that case a new message says "Sign in again to take your previous account or API key off the config file." The claude_authenticate flow used by apps built on the SDK writes this notice to stderr, the error output stream. The result reported when storing the login fails also includes this information.

Why

Switching accounts could leave old credentials in the config file without telling you. Now you are told, and you can sign in again to remove them.