sync-containers3: auto-restart autoswitch on failure
A transient GitHub 5xx while resolving the flake at boot used to leave a
container's autoswitch in a failed oneshot state with no retry, silently
running stale config (observed on yellow: the disk-space guard never
updated to the 100 GiB threshold).
Add Restart=on-failure with RestartSec=120s. Since RestartSec is well
above the default StartLimitIntervalSec, the restart rate limiter never
trips, so a transient failure keeps retrying every 2 min and self-heals
once GitHub recovers.
Assisted-by: Claude Code (claude-opus-4-8[1m])