[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/state/session/index.tsx

@@ -23,16 +23,14 @@ import {readLabelers} from './agent-config'
 
 let __globalAgent: BskyAgent = PUBLIC_BSKY_AGENT
 
-/**
- * NOTE
- * Never hold on to the object returned by this function.
- * Call `getAgent()` at the time of invocation to ensure
- * that you never have a stale agent.
- */
-export function getAgent() {
+function __getAgent() {
   return __globalAgent
 }
 
+export function useAgent() {
+  return React.useMemo(() => ({getAgent: __getAgent}), [])
+}
+
 export type SessionAccount = persisted.PersistedAccount
 
 export type SessionState = {