Cancelled jobs occasionally report an exit status of 255 and nil Signal

I’m tracking down what appears to be a race condition in the job cancellation handling. Jobs are documented to exit with a code of [`128 + SIGNAL`](https://buildkite.com/docs/agent/v3#exit-codes) but I can reproducibly observe cancelled jobs exiting with 255 and no signal. I have also encountered an exit code of `-1`.

No file or test is named. Start by tracing the Go agent's job-cancellation handling and how it derives exit status and signal, then reproduce cancellation while inspecting those values. Done means cancelled jobs consistently report the documented 128 + SIGNAL status rather than 255, nil Signal, or -1.

Written by the indexing model from the issue text.