feat: Add Gemini support and enhance shell completion (zsh only)

This PR introduces ZSH shell completion, improves the Typer CLI structure, and adds support for both OpenAI and Gemini LLMs.

Key changes: • Adds support for Gemini LLM alongside OpenAI • Implements ZSH completion support • Modernizes Typer usage with a typer.Typer app instance • Includes a .gitignore file to exclude python artifacts

Note: I'll pause work on this PR for now. Before merging, would like to implement: • Add tests for the new ZSH completion feature • Ensure compatibility between OpenAI and Gemini models • Confirm OpenAI functionality is fully restored

In the meantime, any feedback is welcome!