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.yml

@@ -0,0 +1,9 @@
+# This file defines Ansible Galaxy dependencies for this repository.
+# It is used by `ansible-galaxy install -r requirements.yml`
+# to download required collections and roles.
+#
+# Since the package-manager project does not rely on Ansible,
+# both sections are intentionally left empty.
+
+collections: []
+roles: []