Refine installer layering and Python/Nix integration

- Introduce explicit CLI layer model (os-packages, nix, python, makefile)
  and central InstallationPipeline to orchestrate installers.
- Move installer orchestration out of install_repos() into
  pkgmgr.actions.repository.install.pipeline, using layer precedence and
  capability tracking.
- Add pkgmgr.actions.repository.install.layers to classify commands into
  layers and compare priorities.
- Rework PythonInstaller to always use isolated environments:
  PKGMGR_PIP override → active venv → per-repo venv under ~/.venvs/<identifier>,
  avoiding system Python and PEP 668 conflicts.
- Adjust NixFlakeInstaller to install flake outputs based on repository
  identity: pkgmgr/package-manager → pkgmgr (mandatory) + default (optional),
  all other repos → default (mandatory).
- Tighten MakefileInstaller behaviour, add global
  PKGMGR_DISABLE_MAKEFILE_INSTALLER switch, and simplify install target
  detection.
- Rewrite resolve_command_for_repo() with explicit Repository typing,
  better Python package detection, Nix/PATH resolution, and a
  library-only fallback instead of raising on missing CLI.
- Update flake.nix devShell to provide python3 with pip and add pip as a
  propagated build input.
- Remove deprecated/wip repository entries from config defaults and drop
  the unused config/wip.yml.

https://chatgpt.com/share/69399157-86d8-800f-9935-1a820893e908

flake.nix

@@ -48,9 +48,7 @@
             # Runtime dependencies (matches [project.dependencies])
             propagatedBuildInputs = [
               pyPkgs.pyyaml
-              # Add more here if needed, e.g.:
-              # pyPkgs.click
-              # pyPkgs.rich
+              pyPkgs.pip
             ];
 
             doCheck = false;
@@ -72,10 +70,16 @@
           ansiblePkg =
             if pkgs ? ansible-core then pkgs.ansible-core
             else pkgs.ansible;
+
+          # Python 3 + pip für alles, was "python3 -m pip" macht
+          pythonWithPip = pkgs.python3.withPackages (ps: [
+            ps.pip
+          ]);
         in
         {
           default = pkgs.mkShell {
             buildInputs = [
+              pythonWithPip
               pkgmgrPkg
               pkgs.git
               ansiblePkg