Add shell autocomplete support for bebe CLI

[Twon]

Description

Implement shell completion for the �ebe CLI using the �rgcomplete library.

Proposed Changes

• Add �rgcomplete to
  equirements.txt.
• Integrate �rgcomplete.autocomplete(parser) into src/bebe/cli.py.
• Document setup instructions for Bash, Zsh, and PowerShell in the README.md.

Background

This was discussed during the initial CLI development and will improve the developer experience when managing multiple build configurations.