lib.Cmd: Add single Cmd to add_subcommands()

add_subcommands() advertises Cmd and list[Cmd] in its signature, but
a single Cmd instance raises NotImplementedError, and since the list
branch handles every element through the same method, a list of Cmd
instances is broken as well. The only working forms are Types and
lists of Types.

Handle a single Cmd instance by reparenting it to the caller and
appending it to the children, tracking its class like the class-based
path does. Instances whose name is already taken by a child are
rejected, mirroring the duplicate-class handling, because argparse
cannot register two subparsers under the same name.

Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.84.2
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/Cmd.py

@@ -88,7 +88,15 @@ class AbstractCmd(abc.ABC):
         self, cmds: Cmd | list[Cmd] | Types[Any] | list[Types[Any]]
     ) -> None:
         if isinstance(cmds, Cmd):
-            raise NotImplementedError('Single Cmd should be handled elsewhere')
+            if any(child.name == cmds.name for child in self.__children):
+                raise Exception(
+                    f'Can\'t register subcommand with already taken name "{cmds.name}"'
+                )
+            self.__child_classes.append(type(cmds))
+            cmds.set_parent(self)
+            self.__children.append(cmds)
+            assert len(self.__children) == len(self.__child_classes)
+            return
         if isinstance(cmds, list):
             for cmd in cmds:
                 self.add_subcommands(cmd)