Refactor GitHappens into modular package architecture

[taherdhanera]

/claim #116

Summary

• split the monolithic gitHappens.py implementation into a githappens/ package with focused modules for config, GitLab API access, git helpers, prompts, templates, and commands
• keep gitHappens.py as the existing executable entrypoint so current aliases continue to work
• add a commands/ package for issue creation, review/time tracking, merge-request opening, incident reports, deploy lookup, and summaries
• document the new structure in the README
• add basic unit tests that lock in the compatibility entrypoint and expected module boundaries

Fixes #116

Testing

• python -m compileall gitHappens.py githappens tests
• python -m unittest discover -s tests -v

[taherdhanera]

Verification note for review:

• python -m compileall gitHappens.py githappens tests
• python -m unittest discover -s tests -v

Both passed locally. The PR keeps the existing gitHappens.py entrypoint in place as a compatibility wrapper, moves implementation into focused modules under githappens/, adds command-specific modules, documents the new structure in the README, and includes structure tests for the refactor.