refactor: split CLI into modular package

[ElRobaMichis]

Summary

Closes #116.

This refactors the monolithic gitHappens.py script into a focused githappens package while keeping the original entry point as a compatibility wrapper.

• moved argument parsing and command routing into githappens/main.py
• moved configuration loading into githappens/config.py
• extracted GitLab API and glab interactions into githappens/gitlab_api.py
• split git helpers, template lookup, prompts, and command workflows into dedicated modules
• added command-specific handlers under githappens/commands/, including create_issue.py, review.py, deploy.py, and open_mr.py
• added unit coverage for config loading, CLI parsing, command workflows, GitLab command construction, git helpers, prompt selection helpers, and templates
• documented the new project structure and test command in the README

Acceptance criteria mapping

• gitlab_api.py: GitLab API calls and glab command construction
• config.py: config.ini and templates.json loading into a typed app context
• templates.py: issue template lookup behavior
• commands/: separate modules for issue creation, review, deploy, MR opening, reporting, and summaries
• existing entry point: gitHappens.py still works as the CLI wrapper
• tests: 18 unit tests across helper modules and command workflows
• README: new project structure and development test command documented

Validation

• python -B -m unittest discover -> 18 tests passing
• python -B gitHappens.py --help
• git diff --cached --check before commit

Prepared with AI assistance and manually validated before submission.