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

mkdocs.yml

@@ -36,14 +36,14 @@ plugins:
 
 nav:
 - Home: docforge/index.md
-- Loader:
-  - docforge/loader/index.md
-  - docforge/loader/griffe_loader.md
-- Model:
-  - docforge/model/index.md
-  - docforge/model/module.md
-  - docforge/model/object.md
-  - docforge/model/project.md
+- Loaders:
+  - docforge/loaders/index.md
+  - docforge/loaders/griffe_loader.md
+- Models:
+  - docforge/models/index.md
+  - docforge/models/module.md
+  - docforge/models/object.md
+  - docforge/models/project.md
 - Navigation:
   - docforge/nav/index.md
   - docforge/nav/spec.md