Resuming a Sprites task whose sandbox was destroyed upstream returned
"sprite not found" and dead-ended the user. Detect the upstream 404 via
a typed ErrSpriteNotFound sentinel, mark the reconnect step skipped with
a user-facing warning explaining what happened, swap the prepare plan
in place to the fresh-provision shape, and continue the launch under a
new sandbox name on the same branch.
The progress plan is now mutable so the reporter can stream the fresh
steps live after the reconnect-then-fresh transition. Only the explicit
not-found case triggers fallback; other reconnect failures still
surface as before, and the fallback fires at most once per launch.
José Almeida