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+

scripts/test/test-integration.sh

@@ -20,7 +20,7 @@ docker run --rm \
     set -e;
     git config --global --add safe.directory /src || true;
     nix develop .#default --no-write-lock-file -c \
-      python -m unittest discover \
+      python3 -m unittest discover \
         -s tests/integration \
         -t /src \
         -p "$TEST_PATTERN";

scripts/test/test-unit.sh

@@ -20,7 +20,7 @@ docker run --rm \
     set -e;
     git config --global --add safe.directory /src || true;
     nix develop .#default --no-write-lock-file -c \
-      python -m unittest discover \
+      python3 -m unittest discover \
         -s tests/unit \
         -t /src \
         -p "$TEST_PATTERN";