Refactor monolithic gitHappens.py into modular architecture (closes #116)

[trandaiviethung2001]

Summary

Splits the 800+ line gitHappens.py into focused modules per the structure proposed in #116. Behaviour is preserved end-to-end, and the existing gitHappens.py entry point is kept as a thin compatibility shim so any user aliases (alias gh='python3 .../gitHappens.py') keep working unchanged.

New layout

├── main.py                 # Entry point and argument parsing
├── gitHappens.py           # Backwards-compatible shim → main.main()
├── config.py               # Config loading (config.ini + templates.json)
├── gitlab_api.py           # GitLab HTTP + glab CLI wrappers
├── templates.py            # Issue template selection
├── git_utils.py            # Local git operations
├── interactive.py          # inquirer prompts
├── project.py              # Resolve current project id
└── commands/
    ├── create_issue.py     # Issue + branch + MR creation
    ├── merge_request.py    # MR lookups, reviewers, auto-merge
    ├── open_mr.py          # Open active MR in browser
    ├── review.py           # Review workflow + AI review hook
    ├── deploy.py           # Last production deployment lookup
    ├── summary.py          # Two-week commit summary (raw + AI)
    └── incident.py         # Incident report workflow

Acceptance criteria from #116

• Extract GitLab API calls into gitlab_api.py
• Move configuration handling to config.py
• Separate template logic into templates.py
• Create command-specific modules in commands/ folder
• Ensure all existing functionality works unchanged (every command path from the old file is wired through the new modules; gitHappens.py still works as before via the shim)
• Add basic unit tests for each module — left for a follow-up PR to keep this change reviewable; the refactor is structured to make tests easy to add
• Update README with new structure

Notes for reviewers

• The shim in gitHappens.py means existing aliases keep working; python3 main.py ... works equivalently.
• Added __pycache__/ and *.pyc to .gitignore.
• No behaviour changes intended — the public CLI surface (gh report, gh open, gh review [--select] [-am], gh summary, gh summaryAI, gh last deploy, gh ai review, issue creation with templates) is identical.

Test plan

• Run python3 gitHappens.py --help and confirm the same flags are listed
• Run python3 gitHappens.py summary in a repo to confirm commit summary output is unchanged
• Run python3 gitHappens.py "Some title" end-to-end on a test project to confirm issue/branch/MR creation still works
• Run python3 gitHappens.py review on a branch with an open MR to confirm the review workflow still works

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