refactor: restructure into src/gitea_ci package

Split monolithic gitea-ci.py (3068 lines) into modular package:
- src/gitea_ci/config.py: configuration, constants
- src/gitea_ci/api.py: GiteaAPI class
- src/gitea_ci/formatters.py: output formatting
- src/gitea_ci/utils.py: utilities, validation
- src/gitea_ci/commands/: command implementations
- src/gitea_ci/cli.py: argument parser, dispatch

gitea-ci.py now thin wrapper (16 lines)

CHEATSHEET.md

@@ -151,3 +151,25 @@ NO_COLOR       Disable colors when set
   "default_repo": "myproject"
 }
 ```
+
+## Package Structure
+
+```
+gitea-ci.py                  Wrapper script (entry point)
+style.py                     Styling utilities
+src/gitea_ci/
+├── cli.py                   Argument parser, dispatch
+├── config.py                Configuration
+├── api.py                   Gitea API client
+├── formatters.py            Output formatting
+├── utils.py                 Utilities
+└── commands/                Command implementations
+```
+
+## Validation
+
+```sh
+python3 -m py_compile src/gitea_ci/*.py     # Syntax check
+./gitea-ci.py --help                        # Test CLI
+python3 -m gitea_ci --help                  # Module mode (PYTHONPATH=src)
+```