refactor: split CLI into modular architecture

[Asaad27]

Summary

• Keep gitHappens.py as the executable compatibility wrapper while moving CLI routing into githappens/main.py.
• Split config loading, GitLab API access, template handling, git utilities, interactive prompts, and command workflows into focused modules.
• Add command modules for issue creation, review, deploy lookup, opening merge requests, and summaries.
• Add focused unittest coverage for the extracted modules and update the README with the new structure.

Verification

• python3 -m unittest discover -s tests -v
• python3 -m compileall gitHappens.py githappens ai_code_review.py
• ./gitHappens.py --help
• git diff --cached --check

Closes #116

AI assistance was used to prepare this PR.