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/models/__init__.pyi

@@ -0,0 +1,9 @@
+from .project import Project
+from .module import Module
+from .object import DocObject
+
+__all__ = [
+    "Project",
+    "Module",
+    "DocObject",
+]