[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/composer/useExternalLinkFetch.e2e.ts

@@ -1,12 +1,14 @@
-import {useState, useEffect} from 'react'
+import {useEffect, useState} from 'react'
+
+import {useAgent} from '#/state/session'
 import * as apilib from 'lib/api/index'
 import {getLinkMeta} from 'lib/link-meta/link-meta'
 import {ComposerOpts} from 'state/shell/composer'
-import {getAgent} from '#/state/session'
 
 export function useExternalLinkFetch({}: {
   setQuote: (opts: ComposerOpts['quote']) => void
 }) {
+  const {getAgent} = useAgent()
   const [extLink, setExtLink] = useState<apilib.ExternalEmbedDraft | undefined>(
     undefined,
   )
@@ -39,7 +41,7 @@ export function useExternalLinkFetch({}: {
       })
     }
     return cleanup
-  }, [extLink])
+  }, [extLink, getAgent])
 
   return {extLink, setExtLink}
 }