refactor(index): surface cause on enoent

[Fdawgs]

Assists in debugging.

TODO: add unit test

Checklist

• Run npm test
• Documentation has been updated and adheres to the style described in CONTRIBUTING.md
• Commit message adheres to the Conventional commits style, following the @commitlint/config-conventional config

[Copilot]

Pull request overview

This PR improves error debugging by preserving the original filesystem error when a file is not found. When an ENOENT error occurs during file opening, the new code wraps it in a "File missing" error with the original error stored in the cause property, making it easier to debug file access issues.

Changes:

• Added error cause chaining to preserve the original ENOENT error when throwing "File missing" errors

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[Copilot]

The test coverage for this change is missing. The existing test at line 385-392 in test/index.test.js only verifies that the error message is "File missing", but doesn't check that the cause property is set correctly with the original ENOENT error. Since the package.json specifies 100% coverage requirements and the PR description mentions "TODO: add unit test", a test should be added to verify the cause property contains the original filesystem error.