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__.py

@@ -0,0 +1,25 @@
+"""
+# Model Layer
+
+The `docforge.models` package provides the core data structures used to represent
+Python source code in a documentation-focused hierarchy.
+
+## Key Components
+
+- **Project**: The root container for all documented modules.
+- **Module**: Represents a Python module or package, containing members.
+- **DocObject**: A recursive structure for classes, functions, and attributes.
+
+These classes are designed to be renderer-agnostic, allowing the same internal
+representation to be transformed into various output formats (currently MkDocs).
+"""
+
+from .project import Project
+from .module import Module
+from .object import DocObject
+
+__all__ = [
+    "Project",
+    "Module",
+    "DocObject",
+]