Fix git unlock on newer Node runtimes

[higgins]

Motivation

• Newer Node runtimes reject non-numeric values passed to process.exit, which caused boolean exit codes to throw and break git clean/smudge filters during clone/unlock workflows.
• Add a regression test to ensure unlocking a cloned repo with an exported key transparently decrypts protected heredocs and to prevent future regressions.

Description

• Replace process.exit(!!msg) with a numeric exit code using process.exit(msg ? 1 : 0) to avoid passing a boolean to process.exit.
• Add an end-to-end regression test test/git-unlock-regression.sh that initializes a repo, commits an encrypted file, clones it, runs git-unlock with the exported key, and verifies decryption.
• Wire npm test to run the new regression script by updating the test script in package.json to bash test/git-unlock-regression.sh.

Testing

• Ran npm test which runs test/git-unlock-regression.sh, and the test completed successfully.

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Codex Task