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/cli/main.py

@@ -8,7 +8,7 @@ from typing import Sequence, Optional
 
 import click
 
-from docforge.loader import GriffeLoader, discover_module_paths
+from docforge.loaders import GriffeLoader, discover_module_paths
 from docforge.renderers.mkdocs import MkDocsRenderer
 from docforge.cli.mkdocs import mkdocs_cmd