feat: add agent marketplace repository and pin sub-agent versions at publish

Introduce ag_agent_repository_t with list/status/publish/import APIs for
frozen agent snapshots. Pin selected_agent_version_no on agent relations when
publishing so sub-agents resolve to a fixed version at runtime. Extend agent
export/import to bundle skills in ZIP payloads and add embedding model fallback
when no model name is provided.

Author: Lifeng-Chen

test/backend/app/test_idata_app.py

@@ -517,10 +517,10 @@ def test_router_prefix(self):
     def test_routes_registered(self):
         """Test that all routes are registered."""
         app = _build_app()
-        routes = [route.path for route in app.routes]
+        paths = app.openapi()["paths"]
 
-        assert "/idata/knowledge-space" in routes
-        assert "/idata/datasets" in routes
+        assert "/idata/knowledge-space" in paths
+        assert "/idata/datasets" in paths
 
     def test_router_methods(self):
         """Test that routes have correct HTTP methods."""

test/backend/app/test_northbound_base_app.py

@@ -274,8 +274,8 @@ def test_cors_middleware_configuration(self):
 
     def test_router_inclusion(self):
         """The main northbound router should be included."""
-        routes = [route.path for route in app.routes]
-        self.assertIn("/dummy", routes)
+        paths = app.openapi()["paths"]
+        self.assertIn("/dummy", paths)
 
     def test_a2a_router_inclusion(self):
         """A2A router should be registered under /nb/a2a."""