Refine Nix dev shell, ensure PyYAML availability, fix Python invocation, and

expose pkgmgr.cli for Python 3.13 compatibility

- Add `.nix-dev-installed` to .gitignore
- Improve flake.nix:
  * unify pkgs/pyPkgs definitions
  * provide python311.withPackages including pip + PyYAML
  * remove unused pkgmgrPkg reference from devShell
  * fix PYTHONPATH export and devShell help message
- Update unit/integration test scripts to use `python3 -m unittest`
- Add top-level pkgmgr.__init__ exposing `cli` attribute for
  pkgutil.resolve_name compatibility under Python 3.13+

src/pkgmgr/__init__.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+"""
+Top-level package for Kevin's package manager (pkgmgr).
+
+We re-export the CLI subpackage as the attribute ``cli`` so that
+``pkgutil.resolve_name("pkgmgr.cli.commands.release")`` and similar
+lookups work reliably under Python 3.13+.
+"""
+
+# Re-export the CLI subpackage as an attribute on the package.
+from . import cli as cli  # type: ignore[F401]
+
+__all__ = ["cli"]