The pool stopped retrying only on exit 0 or ExitPermanent (76), so the
other codes `zero exec` uses to say a run is over (2 usage, 3 provider,
4 incomplete, 130 interrupted) were treated as crashes and retried up to
MaxAttempts with backoff. Exit 4 means the agent already ran, so each
retry repeated its side effects, re-billed the provider and appended a
second run to the same session stream.
Treat those four codes as final: return the worker's exit code with
ErrPermanent. Crash-type exits (1, signals) and launch/read errors still
retry.
Fixes#1100
Co-authored-by: Claude Sonnet 5.5 <[email protected]>
