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

README.md

@@ -56,7 +56,7 @@ doc-forge server mypackage
 ### Python API
 
 ```python
-from docforge.loader import GriffeLoader
+from docforge.loaders import GriffeLoader
 from docforge.renderers import MkDocsRenderer
 from pathlib import Path
 
@@ -70,6 +70,7 @@ renderer.generate_sources(project, Path("docs"))
 
 # Build final documentation
 from docforge.renderers.base import RendererConfig
+
 config = RendererConfig(Path("docs"), project)
 renderer.build(config)
 ```