♻️ Refactor git module into structured submodules

Split monolithic git.rs into specialized submodules

This change improves code organization and maintainability by:

- Creating dedicated modules for commits, files, metadata, and repository
- Establishing clear separation of concerns between components
- Maintaining backward compatibility through re-exports
- Preserving all existing functionality while improving structure
- Adding proper module documentation and organization

The refactoring makes the codebase more maintainable and easier to
extend with new Git-related features.

Author: hyperb1iss