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>

app_skellington/_version.py

@@ -1,24 +0,0 @@
-# file generated by vcs-versioning
-# don't change, don't track in version control
-from __future__ import annotations
-
-__all__ = [
-    "__version__",
-    "__version_tuple__",
-    "version",
-    "version_tuple",
-    "__commit_id__",
-    "commit_id",
-]
-
-version: str
-__version__: str
-__version_tuple__: tuple[int | str, ...]
-version_tuple: tuple[int | str, ...]
-commit_id: str | None
-__commit_id__: str | None
-
-__version__ = version = '0.3.0.dev10+gda9568b4f.d20261005'
-__version_tuple__ = version_tuple = (0, 3, 0, 'dev10', 'gda9568b4f.d20261005')
-
-__commit_id__ = commit_id = 'gda9568b4f'