feat(mcp): add MCP JSON renderer and CLI support, update tests accordingly

- Add MCPRenderer to generate MCP-native JSON bundles (index.json, nav.json, modules/*.json)
- Expose MCPRenderer via public API and CLI (`generate-mcp` command)
- Replace Markdown-based MCP output with structured JSON resources
- Update MCP renderer type stubs to match new JSON-based implementation
- Refactor MCP tests to validate JSON content, bundle structure, and navigation
- Fix MCP module coverage test to use explicit project_root for reliable discovery

tests/renderers/mcp/test_mcp_content.py

@@ -1,3 +1,4 @@
+import json
 from pathlib import Path
 
 from docforge import MCPRenderer
@@ -13,6 +14,8 @@ def test_mcp_file_content(tmp_path: Path):
 
     renderer.generate_sources(project, out_dir)
 
-    content = (out_dir / "testpkg" / "mod.md").read_text()
+    module_file = out_dir / "modules" / "testpkg.mod.json"
+    payload = json.loads(module_file.read_text())
 
-    assert "# Module `testpkg.mod`" in content
+    assert payload["module"] == "testpkg.mod"
+    assert payload["content"]["path"] == "testpkg.mod"