Align Nix Python environment and add lazy CLI import

- Switch flake package and dev shell to Python 3.11 to match pyproject
- Ensure the python-with-deps environment is preferred on PATH in nix develop
- Introduce a lightweight pkgmgr __init__ with lazy loading of pkgmgr.cli
- Avoid pulling in CLI/config dependencies on plain `import pkgmgr`, fixing
  unit test imports and PyYAML availability in the Nix test containers

https://chatgpt.com/share/693a9723-27ac-800f-a6c2-c1bcc91b7dff

src/pkgmgr/__init__.py

@@ -0,0 +1,36 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""
+Top-level pkgmgr package.
+
+We deliberately avoid importing heavy submodules (like the CLI)
+on import to keep unit tests fast and to not require optional
+dependencies (like PyYAML) unless they are actually used.
+
+Accessing ``pkgmgr.cli`` will load the CLI module lazily via
+``__getattr__``. This keeps patterns like
+
+    from pkgmgr import cli
+
+working as expected in tests and entry points.
+"""
+
+from __future__ import annotations
+
+from importlib import import_module
+from typing import Any
+
+__all__ = ["cli"]
+
+
+def __getattr__(name: str) -> Any:
+  """
+  Lazily expose ``pkgmgr.cli`` as attribute on the top-level package.
+
+  This keeps ``import pkgmgr`` lightweight while still allowing
+  ``from pkgmgr import cli`` in tests and entry points.
+  """
+  if name == "cli":
+      return import_module("pkgmgr.cli")
+  raise AttributeError(f"module 'pkgmgr' has no attribute {name!r}")