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

flake.nix

@@ -0,0 +1,40 @@
+# flake.nix
+# This file defines a Nix flake providing a reproducible development environment
+# and optional installation package for the package-manager tool.
+
+{
+  description = "Nix flake for Kevin's package-manager tool";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in {
+
+      # Development environment used via: nix develop
+      devShells.default = pkgs.mkShell {
+        # System packages for development
+        buildInputs = [
+          pkgs.python311
+          pkgs.python311Packages.pyyaml
+          pkgs.git
+        ];
+
+        # Message shown on environment entry
+        shellHook = ''
+          echo "Entered pkgmgr development environment";
+        '';
+      };
+
+      # Optional installable package for "nix profile install"
+      packages.pkgmgr = pkgs.python311Packages.buildPythonApplication {
+        pname = "package-manager";
+        version = "0.1.0";
+        src = ./.;
+        propagatedBuildInputs = [ pkgs.python311Packages.pyyaml ];
+      };
+    };
+}