cfg (ast.SandboxV2) is a pointer shared across all concurrent calls to
the same Assistant. GetComputer writes cfg.ID = <random hex> for each
oneshot request, so concurrent tasks overwrite each other's ID.
When cleanup runs in defer, cfg.ID holds whichever task last wrote it —
not the ID of the box this request created — so manager.Remove/Heartbeat
silently fails with "sandbox: not found" and the container leaks.
Fix: derive the box ID from computer.ComputerInfo().BoxID, which is the
immutable b.id set at Create time and never mutated by concurrent callers.
Made-with: Cursor
cc0bb7e161