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.nav.yml

@@ -1,13 +1,13 @@
 home: docforge/index.md
 groups:
-  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