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/loaders/griffe_loader.pyi

@@ -0,0 +1,31 @@
+from typing import List, Optional
+from pathlib import Path
+
+from docforge.models import Module, Project
+
+
+def discover_module_paths(
+    module_name: str,
+    project_root: Path | None = None,
+) -> List[str]:
+    ...
+
+
+class GriffeLoader:
+    """Griffe-based introspection loaders.
+
+    This is the only supported introspection backend in doc-forge.
+    """
+
+    def __init__(self) -> None: ...
+
+    def load_project(
+        self,
+        module_paths: List[str],
+        project_name: Optional[str] = ...,
+        skip_import_errors: bool = ...,
+    ) -> Project:
+        """Load a documentation project from Python modules."""
+
+    def load_module(self, path: str) -> Module:
+        """Load a single Python module by import path."""