refactor: rename loader/model packages to loaders/models

- Rename docforge.loader → docforge.loaders and docforge.model → docforge.models
- Update all imports, type stubs, CLI, tests, and documentation references
- Align MkDocs navigation and docforge.nav.yml with new package structure
- Adjust module docstrings and comments for consistency with pluralized naming

docforge/__init__.pyi

@@ -1,7 +1,7 @@
-from .loader import GriffeLoader, discover_module_paths
+from .loaders import GriffeLoader, discover_module_paths
 from .renderers import MkDocsRenderer
 from .cli import main
-from . import model
+from . import models
 
 __all__ = [
     "GriffeLoader",