Keep multiple Runtype accounts in the CLI

The CLI can now keep separate logins for multiple organizations. Use runtype auth accounts to see them, auth switch to change your default, or runtype link to bind a project directory to its organization.

A linked project will not silently use another organization's login. Project links contain no secrets and can live in source control. Existing CLI credentials migrate on first use.

Credentials use your operating system's keychain when available. Headless environments can use the supported file-store fallback instead.