Refactors the CLI argument parsing to use Python's argparse module instead of manual sys.argv parsing.

Benefits:

• More robust argument handling
• Better help messages (automatically formatted)
• Easier to extend with additional arguments in the future
• Standard Python practice for CLI applications

Inspired by PR #12 from @plars. Rather than requiring a rebase of that months-old PR, I've incorporated the argparse suggestion directly while crediting Paul in the commit.

No functional changes - the CLI behavior remains the same.