Create commit0-core package

[xingyaoww]

This PR creates a new package commit0-core that separates the core functionality from aider-chat dependencies. Changes include:

1. Created commit0-core package containing:

   • All harness functionality
   • CLI interface
   • Core dependencies (ruff, pre-commit, etc.)

2. Updated original commit0 package to:

   • Depend on commit0-core==0.1.6
   • Keep only the aider-chat dependency
   • Import functionality from commit0-core

This separation allows users to:

• Install just the core functionality: pip install commit0-core
• Install the full package with aider-chat: pip install commit0