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

.gitignore

@@ -1,2 +1,27 @@
+
+# Prevents unwanted files from being committed to version control.
+
+# Custom Config file
 config/config.yaml
-*__pycache__*
+
+# Python bytecode
+__pycache__/
+*.pyc
+
+# Virtual environments
+.venv/
+venv/
+.venvs/
+
+# Build artifacts
+dist/
+build/
+*.egg-info/
+
+# Editor files
+.vscode/
+.idea/
+
+# OS noise
+.DS_Store
+Thumbs.db