feat: package dockreap as proper Python CLI and update README

- Add pyproject.toml for setuptools-based packaging
- Rename entrypoint to dockreap.__main__ and expose console script
- Update README with pip/pipx installation instructions
- Remove obsolete package-manager references
- Add MIRRORS file including PyPI endpoint
- Align documentation with actual volume usage detection logic

MIRRORS

@@ -0,0 +1,4 @@
+git@github.com:kevinveenbirkenbach/dockreap.git
+ssh://git@code.infinito.nexus:2201/kevinveenbirkenbach/dockreap.git
+git@github.com:kevinveenbirkenbach/dockreap.git
+https://pypi.org/project/dockreap/