Make agent dependencies optional

[xingyaoww]

This PR adds a core installation option that excludes agent-related dependencies. Changes include:

1. Keep aider-chat in main dependencies (so pip install commit0 installs everything)
2. Add core optional dependencies group that excludes aider-chat
3. Move agent package to commit0/optional/agent
4. Add build hook to include/exclude agent package based on installation type

This allows users to:

• Install everything (default): pip install commit0
  • Includes agent package
  • Includes aider-chat dependency
  • Provides commit0 command
• Install without agent support: pip install commit0[core]
  • Excludes agent package
  • Excludes aider-chat dependency
  • Provides same commit0 command

This is a simpler approach than previous PRs as it:

• Keeps all code in the same package
• Uses Python's built-in optional dependencies
• Makes the full installation the default
• Uses the same command (commit0) for both installations

[wenting-zhao]

merged in #96