GRM-20: refactor(scripts): centralize constants, API clients, and HTTP status codes

- Add shared config.py with API URLs, regexes, timeouts, pagination
- Add GiteaClient and VikunjaClient in api_clients.py with pooled sessions
- Add APIError exception for unified HTTP error handling
- Refactor all scripts to use shared modules and http.HTTPStatus
- Rewrite unit tests to mock clients and use HTTPStatus constants
- Add tests for api_clients and config modules
- Achieve 100% test coverage

tests/unit/test_validate_commit_msg.py

@@ -7,7 +7,8 @@ from unittest.mock import patch
 
 from click.testing import CliRunner
 
-from scripts.validate_commit_msg import CONVENTIONAL_RE, TASK_ID_RE, first_line, get_branch, main
+from gitea_runner_manager.config import CONVENTIONAL_RE, TASK_ID_RE
+from scripts.validate_commit_msg import first_line, get_branch, main
 
 
 class TestHelpers: