build: uv dependency groups + lockfile; untrack generated _version.py

- dev tools move from a published extra to [dependency-groups]
- uv.lock resolves one lock across CPython 3.8-3.14
- pytest config moves into pyproject (testpaths)
- explicit packages list; license classifier corrected to MIT-0
- drop obsolete setup.cfg and tests/pytest.ini

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -3,3 +3,5 @@ dist
 *.egg-info
 __pycache__
 .idea
+.venv/
+app_skellington/_version.py