build: derive versions from SCM and gate PyPI publishing

[justusschock]

What

• derive package versions from Git tags with setuptools-scm
• expose the installed distribution version through litlogger.__version__
• publish to PyPI only for a published GitHub Release using OIDC Trusted Publishing, without stored PyPI credentials

Verification

• Python 3.11 sdist and wheel build with setuptools>=80
• isolated wheel install/import with litlogger.__version__ matching package metadata
• ruff check src/litlogger/__init__.py
• workflow YAML parse and publish-condition assertion
• git diff --check