tuist setup cache no longer pins a cache-start respawn loop when logged out

Running tuist setup cache while logged out no longer installs a KeepAlive launchd agent that respawns cache-start every ~10 seconds and accumulates empty session directories. The daemon now exits cleanly (exit 0) when no credential is available, the LaunchAgent KeepAlive is now SuccessfulExit=false so launchd only restarts on a crash, and setup itself fails fast with an actionable auth error before installing anything. CI workflows using TUIST_TOKEN are unaffected.