Refactor CLI into modular architecture

[sailxjx]

Summary

• Split the monolithic gitHappens.py into focused modules for config loading, Git helpers, GitLab API integrations, template handling, interactive prompts, and command workflows.
• Kept gitHappens.py as the CLI entry point and preserved the existing command dispatch behavior.
• Added basic pytest coverage for the extracted modules and documented the new project structure and test command in the README.

Testing

• .venv/bin/python -m pytest
• .venv/bin/python -m py_compile gitHappens.py config.py git_utils.py gitlab_api.py templates.py interactive.py commands/*.py ai_code_review.py
• .venv/bin/python gitHappens.py --help

Closes #116