Implement arithmetic functions + docstrings

[srajolu-glitch]

Summary

• Implemented add_numbers, factorial, and is_prime in src/arithmetic/arithmetic.py
• Added module/function docstrings to satisfy pydocstyle (ruff) rules
• All tests pass locally with `poetry run pytest -q
• Pre-commit hooks pass (formatting, mypy, ruff, etc.)

How to test

1. poetry install`
2. poetry run pre-commit run --all-files # all green
3. poetry run pytest -q # tests pass

Notes

• Branch: feature/siddendra_suryateja
• Ready for code review.