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/module.pyi

@@ -0,0 +1,23 @@
+from typing import Dict, Iterable, Optional
+
+from docforge.models.object import DocObject
+
+
+class Module:
+    """Represents a documented Python module."""
+
+    path: str
+    docstring: Optional[str]
+    members: Dict[str, DocObject]
+
+    def __init__(
+        self,
+        path: str,
+        docstring: Optional[str] = ...,
+    ) -> None: ...
+
+    def add_object(self, obj: DocObject) -> None: ...
+
+    def get_object(self, name: str) -> DocObject: ...
+
+    def get_all_objects(self) -> Iterable[DocObject]: ...