Restarter: skip user/concurrency cancellations, request restarts on unregister

[AndWeHaveAPlan]

• Cancelled runs are restarted unless a job annotation shows a person or a concurrency group cancelled them (needs GitHub App "Checks: read"; 403 fails open)
• /agent/interrupt ignores trays without a job: idle preempted trays were creating run-id-0 requests that hit GitHub with a 404 every 30s for 6h
• Unregister with reason Preempted or SigTerm requests the restart itself, before the tray is deleted. Covers the case where the VM's shutdown handler loses the race with systemd and never posts the interrupt