chore: exclude unnecessary files from gem package

[willnet]

Replace git ls-files with explicit file list to avoid including test files, development configs, and build artifacts in gem.

Only include lib/ directory and essential documentation.

Also, remove test_files since it doesn’t seem to be included in gemspec attributes. ref: https://guides.rubygems.org/specification-reference/

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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