Package pkgmgr with multi-format manifests

- Add pyproject.toml and setuptools config for pkgmgr packaging
- Add Nix flake (devShell + pkgmgr package output)
- Add Arch PKGBUILD for system packaging
- Introduce pkgmgr.yml manifest for repo-level dependencies
- Refactor CLI into pkgmgr/cli.py and make main.py a thin entrypoint
- Extend install_repos to handle pkgmgr.yml, PKGBUILD, flake.nix, Ansible and Python manifests
- Enhance status/update to show Nix/yay system status and upgrades
- Improve .gitignore and document requirements.yml

Created with AI (ChatGPT) – see conversation: https://chatgpt.com/share/6932f2ca-f560-800f-8bb0-52cb82f27e88

requirements.txt

@@ -1 +1,4 @@
-PyYAML
+# Legacy file used only if pip still installs from requirements.txt.
+# You may delete this file once you switch entirely to pyproject.toml.
+
+PyYAML