sandbox: out-of-band computer status and restart via the execute tool (#469)

When a sandbox wedges (the guest stops answering, then every exec fails instantly), the agent's only signal is hanging commands and its only remedy is a human operator restarting the machine by hand. The execute tool gains an optional computer: "status" | "restart" parameter that acts out-of-band on the scoped computer's control plane, so it works precisely when the box can't run anything: status combines the provider's health-check endpoint with a shell probe (surfacing the "machine healthy, shell not answering" wedge signature), and restart reboots the machine — files survive, running processes don't, and the tool result says an interrupted command may or may not have taken effect. The Sandbox interface gains optional computerStatus/restartComputer hooks; backends that don't implement them return a clear capability-unsupported error. No automatic restart policy is baked in — the agent decides, using signals it can observe.

**Deployment notes**

Behavior flip: a stalled sprite exec channel now triggers an automatic machine restart (deduped,
2-min cooldown) instead of hanging to deadline — a live sandbox can now be rebooted by the
platform without operator action; surfaced via onError as sprite_restarted
Restart cooldown state is in-memory per instance, so blue-green can at worst issue one restart
per instance — bounded and acceptable; no schema/config changes

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