tuist setup cache fails fast when not authenticated

Running tuist setup cache while logged out is no longer a silent footgun. The setup command now fails fast with an actionable error if the user is not authenticated (or TUIST_TOKEN is not set), instead of installing a LaunchAgent whose daemon would exit immediately for lack of credentials. The CacheStart agent itself exits cleanly on a missing credential (with the LaunchAgent KeepAlive polly set to SuccessfulExit = false) and stops the previous ~10-second respawn storm that left empty session directories behind.