feat: add agent registry manifests and coverage

Introduce a manifest-backed agent registry surface and align graph tests with the new runtime prompt and tool indexing behavior.

backend/app/agents/registry/__init__.py

@@ -0,0 +1,11 @@
+"""Registry manifest models and validation helpers."""
+
+from app.agents.registry.indexes import RegistryIndexes, build_registry_indexes
+from app.agents.registry.loader import RegistryBundle, load_builtin_registry_bundle
+
+__all__ = [
+    "RegistryBundle",
+    "RegistryIndexes",
+    "build_registry_indexes",
+    "load_builtin_registry_bundle",
+]