refactor(cli): replace deprecated click.MultiCommand with click.Group

MultiCommand is deprecated since Click 8.3 and removed in 9.0; its functionality was folded into Group, which supports the same list_commands/get_command lazy-loading overrides. Unblocks a future Click 9 bump and silences the import-time DeprecationWarning that broke test collection under -W error::DeprecationWarning.

unshackle/core/commands.py

@@ -59,7 +59,7 @@ def check_load_errors() -> None:
         raise click.ClickException(f"Failed to load {len(LOAD_ERRORS)} command(s):\n{joined}")
 
 
-class Commands(click.MultiCommand):
+class Commands(click.Group):
     """Lazy-loaded command group of project commands."""
 
     def list_commands(self, ctx: click.Context) -> list[str]:

unshackle/core/services.py

@@ -76,7 +76,7 @@ def check_load_errors() -> None:
         raise click.ClickException(f"Failed to load {len(LOAD_ERRORS)} service(s):\n{joined}")
 
 
-class Services(click.MultiCommand):
+class Services(click.Group):
     """Lazy-loaded command group of project services."""
 
     _remote_services_cache: list[dict] | None = None