fix(cli): gate silent-abort on signal handler firing; test exit codes

Addresses two trail findings:

• Matching context.Canceled at the top level was too broad: an internally-cancelled sub-context would have silently re-raised SIGINT (exit 130) and broken an enclosing loop even without a Ctrl-C. Gate the abort path on caughtSignal being set, so a signal-less context.Canceled falls through to normal error reporting.
• Add a table test for exitCodeForSignal locking SIGINT→130, SIGTERM→143 (and the non-numeric fallback), guarding the mapping the fix relies on.