fix: reject signal-terminated drop-sudo commands

[original4422]

Summary

Treat child processes terminated by a signal as failed drop-sudo commands instead of normalizing their null exit code to zero.

Fixes #152.

Root cause

Node reports code === null and supplies the terminating signal separately for a signal-terminated child. The shared execCommand() helper used code ?? 0, so required privilege-transition commands could resolve successfully after interruption.

Changes

• preserve the nullable child exit code;
• reject required commands when code is null and include the signal in the diagnostic;
• retain the exit code and signal on the structured error;
• add a mocked regression for close(null, "SIGTERM");
• rebuild the checked-in dist/main.js bundle.

Validation

• Regression test failed on unmodified main with Missing expected rejection.
• node --test test/linuxCredentials.test.mjs — 5 passed.
• pnpm run check — passed.
• pnpm test — 145 tests: 142 passed, 3 platform-dependent tests skipped on macOS.
• git diff --check — passed.

Risk

Low. Numeric exit handling is unchanged. Signal termination is now a failure for required commands and remains non-throwing only at call sites that explicitly set ignoreFailure.

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[original4422]

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