refactor(agents): replace useMutation with simple function in useAddAgent

The mutation logic was removed and replaced with a simple function that shows a toast message. This is a temporary solution until the actual API implementation is ready.

src/renderer/src/hooks/agents/useAddAgent.ts

@@ -1,16 +1,7 @@
-import { AgentEntity } from '@renderer/types'
-import { useMutation, useQueryClient } from '@tanstack/react-query'
-
 export const useAddAgent = () => {
-  const qc = useQueryClient()
-
-  // TODO: use api
-  return useMutation({
-    mutationFn: async (agent: AgentEntity) => {
-      return agent
-    },
-    onSuccess: (added: AgentEntity) => {
-      qc.setQueryData<AgentEntity[]>(['agents'], (old) => (old ? [...old, added] : [added]))
+  return {
+    addAgent: () => {
+      window.toast.info('Not implemented')
     }
-  })
+  }
 }