fix: Propagate correct exit codes on signal termination (Unix)

[insign]

This PR improves the CLI's handling of processes that are killed by signals. It maps None exit codes to standard shell-compatible exit codes (128 + signal) on Unix platforms, allowing callers to correctly interpret termination causes (e.g., distinguishing between a clean failure exit 1 and a signal termination kill -9).

This is a valuable improvement for developers using run within automated scripts, system services, or continuous integration environments where reliable signal propagation is expected behavior.

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PR created automatically by Jules for task 25870807027458885 started by @insign

[google-labs-jules]

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