ADR-021: Persistent auth configuration and keyring storage

bb auth login stores a host: its profile (URL, aliases, username, auth mode and client certificate paths) in the user's configuration file, and its token or password in the OS keyring, or, where no keyring can hold it and the login asks with --allow-insecure-storage, in the file (ADR-047). bb keeps several hosts, one of them the default. A flag or an environment variable outranks what is stored, for the one invocation it belongs to: a flag's value is passed down that call as an override and never written into the process environment, where it would replace the user's own value and outlive the call. The whole order, with the workspace and system files, is ADR-058.

A stored credential is released only to the host it was stored for: that host, an alias of it (ADR-041), or the same host stored over http:// and reached over https://. One stored for https:// is never sent over http://, and a host with nothing stored for it gets no credential, never the default host's. Only the user's own file and the system file decide that: a host profile in the workspace file, which arrives with a clone, gives a host its url and username, never makes bb release a stored credential, and its client certificate is ignored. Resolve credentials through the configuration load or config.LoadStoredAuthForHostStrict, which hold to this, and do not add a lookup that falls back to the default host.

Logging in once per host beats exporting variables in every shell, and the keyring keeps the secret off disk. The host a command talks to can still be chosen by inputs an attacker reaches, though a repository's files do not choose it (ADR-058): a URL given to bb api, --host on an MCP server a prompt-injected agent drives. A fallback hands the user's token to whichever host they name, and a scheme they downgrade sends it across the network in the clear. A credential stored over http:// answering over https:// adds TLS rather than removing it, so a login made before the server had a certificate keeps working.

Not chosen

• Plaintext secrets in the configuration file: Puts every credential on disk; the file holds a secret only where no keyring can and the user asked for it.