Stop cache-start daemon respawn loop when unauthenticated

Running tuist setup cache while logged out no longer installs a background daemon that respawns every ~10 seconds and creates a new session directory each time. cache-start now exits cleanly when no credential is available, the LaunchAgent only restarts on crash, and tuist setup cache fails fast with a clear tuist auth login message instead of silently appearing to succeed. CI flows that use TUIST_TOKEN are unaffected.