refactor: split CLI into modular package

[rtmalikian]

Summary

• Split the monolithic gitHappens.py entrypoint into a githappens/ package with focused config, GitLab API, git utility, template, interactive prompt, and command modules.
• Kept gitHappens.py as a backward-compatible executable wrapper for existing aliases.
• Added unit tests covering parser dispatch, config/template loading, GitLab command construction, report incident issue type preservation, and command module behavior.
• Updated the README with the new project structure.

OpenPledge

• Funded issue: https://app.openpledge.io/issues/95
• GitHub issue: Refactor monolithic gitHappens.py into modular architecture #116

Verification

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s tests -v
• PYTHONDONTWRITEBYTECODE=1 python3 gitHappens.py --help
• git diff --check

Closes #116