feat: add --api OpenAPI build mode with docs/api scheme and lib-prefixed nav

docforge/cli/api_utils.pyi

@@ -0,0 +1,13 @@
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+@dataclass
+class OpenAPIMetadata:
+    site_name: str
+    site_description: str | None
+    site_author: str | None
+
+def load_openapi_spec(spec_path: Path) -> dict[Any, Any]: ...
+def derive_metadata(spec: dict[Any, Any]) -> OpenAPIMetadata: ...
+def generate_api_sources(spec: dict[Any, Any], docs_dir: Path) -> None: ...