chore(agent_api_keys): update openapi spec

Author: danielmillerp

agentex/openapi.yaml

@@ -4615,7 +4615,9 @@ components:
           - type: string
           - type: 'null'
           title: Secret
-          description: Optional signing secret; if unset, one is generated and returned.
+          description: Signing secret. For GitHub, omit to generate one, or provide
+            an existing webhook secret. For Slack, this is required and must be the
+            Slack app's Signing Secret.
         base_url:
           anyOf:
           - type: string