[Refactor] Use relativePath utility in copyDirectoryContents

[gonzaloriestra]

This PR refactors the copyDirectoryContents function in the cli-kit package to improve its readability and robustness.

• Replaced manual .replace() calls with the relativePath utility for consistent path calculation.
• Switched from an imperative loop with an array push to a more idiomatic .map() for collecting file-copy promises.
• Avoided variable shadowing by using relPath as the local variable name.

Verified with existing tests in packages/cli-kit/src/public/node/fs.test.ts.

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PR created automatically by Jules for task 12107833890165445825 started by @gonzaloriestra

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