[Session] Add useAgent hook and replace (#3706)

* Hook it up

* Memoize getAgent method

* Use one shared reference

---------

Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

src/view/com/modals/ChangeHandle.tsx

@@ -16,8 +16,8 @@ import {useModalControls} from '#/state/modals'
 import {useFetchDid, useUpdateHandleMutation} from '#/state/queries/handle'
 import {useServiceQuery} from '#/state/queries/service'
 import {
-  getAgent,
   SessionAccount,
+  useAgent,
   useSession,
   useSessionApi,
 } from '#/state/session'
@@ -40,6 +40,7 @@ export type Props = {onChanged: () => void}
 
 export function Component(props: Props) {
   const {currentAccount} = useSession()
+  const {getAgent} = useAgent()
   const {
     isLoading,
     data: serviceInfo,