fix: strip export lines from ref files in pre-commit hook

[rickvian]

Summary

• Fixed pre-commit hook (build_injected_file) to strip export lines from reference solution files before injecting them into the playground file
• Without this fix, ref files that already contain export { fn }; (like 0424-longest-repeating-character-replacement.js) would produce a duplicate export statement, crashing the module and blocking every commit

Test plan

• Stage a test file for a problem whose ref solution has an export line (e.g. 0424-longest-repeating-character-replacement)
• Run git commit — hook should report OK instead of blocking with a module error
• Verify the playground stub is restored after the hook runs

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