Add commit0-core package configuration

[xingyaoww]

This PR creates a commit0-core package that provides the core functionality without agent-related features. Changes include:

1. Created commit0-core/ directory with:

   • Core package code (copied from commit0)
   • Excluded agent package
   • Core dependencies only (no aider-chat)
   • Separate pyproject.toml and README.md

2. Original commit0 package remains unchanged

This allows users to:

• Install just the core functionality: pip install commit0-core
• Install the full package with agent support: pip install commit0

Note: This is a simpler approach than the previous PR (#92) as it keeps the original package unchanged and creates a separate package for core functionality.

[xingyaoww]

supersede by #95