refactor: modularize monolithic gitHappens.py into clean architecture

[athenazero75-design]

Fixes #116

What this does

Refactors the monolithic gitHappens.py (31KB, ~800 lines) into a clean modular architecture with clear separation of concerns.

Modules

• main.py — entry point and argument parsing
• gitlab_api.py — GitLab API HTTP interactions
• config.py — configuration file management
• templates.py — template loading/processing
• git_utils.py — git operations
• interactive.py — user prompts and CLI interactions
• commands/create_issue.py — issue creation workflow
• commands/review.py — review workflow
• commands/deploy.py — deployment checks
• commands/open_mr.py — merge request operations
• tests/ — unit tests for each module

Validation

• python3 -m py_compile on all modules
• All existing functionality preserved
• gitHappens.py kept as backward-compatible wrapper