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)

.gitea/workflows/ci.yml

@@ -28,10 +28,15 @@ jobs:
         run: pip install -q ruff
 
       - name: Syntax check
-        run: python -m py_compile gitea-ci.py style.py
+        run: |
+          python -m py_compile gitea-ci.py style.py
+          python -m py_compile src/gitea_ci/*.py src/gitea_ci/commands/*.py
 
       - name: Lint
-        run: ruff check gitea-ci.py style.py
+        run: ruff check gitea-ci.py style.py src/gitea_ci/
 
       - name: Test --help
         run: python gitea-ci.py --help
+
+      - name: Test import
+        run: python -c "from src.gitea_ci import Config, GiteaAPI, __version__; print(f'v{__version__}')"