migrate the remaining agent reads outside the session layer

The last non-session `useAgent` consumers, all thin aliases over calls the
clients already make:

- `handle.ts`: `getProfile` / `resolveHandle` to the appview client.
- `post.ts`: the three post readers share one `fetchPost` helper on the appview
  client. Their consumers still want `@atproto/api` views, so the generated view
  is asserted across at that single boundary rather than at each call site.
- `post-feed.ts`: `agent.session` gated the logged-out "did any post survive
  moderation" assertion, which is a question about the session, not the
  transport - it reads `hasSession` from the session context now.
- `ChangeHandleDialog`: `agent.serviceUrl` becomes `currentAccount.service`.
- `TestCtrls.e2e`: drops the `configureProxy` call. The appview client reads
  `BLUESKY_PROXY_HEADER` when the bundle builds it, and the gate around this
  input means no bundle exists yet, so setting the constant is sufficient.

`preferences/index.ts` still writes labeler subscriptions to the agent; that
one moves with the bundle rework, which is what gives it a client to write to.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/view/com/testing/TestCtrls.e2e.tsx

@@ -3,7 +3,7 @@ import {LogBox, Pressable, TextInput, View} from 'react-native'
 import {useQueryClient} from '@tanstack/react-query'
 
 import {BLUESKY_PROXY_HEADER} from '#/lib/constants'
-import {useAgent, useSessionApi} from '#/state/session'
+import {useSessionApi} from '#/state/session'
 import {useLoggedOutViewControls} from '#/state/shell/logged-out'
 import {useOnboardingDispatch} from '#/state/shell/onboarding'
 import {navigate} from '../../../Navigation'
@@ -31,7 +31,6 @@ const BTN = {height: 1, width: 1, backgroundColor: 'red'}
 let hasConfiguredProxy = false
 
 export function TestCtrls() {
-  const agent = useAgent()
   const queryClient = useQueryClient()
   const {logoutEveryAccount, login} = useSessionApi()
   const onboardingDispatch = useOnboardingDispatch()
@@ -74,8 +73,13 @@ export function TestCtrls() {
         autoCapitalize="none"
         onSubmitEditing={() => {
           const header = `${proxyHeader}#bsky_appview`
+          /*
+           * The appview client reads `BLUESKY_PROXY_HEADER.get()` when the
+           * bundle builds it (see clients.ts), so setting the mutable constant
+           * retargets the proxy for the sign-ins below without reconfiguring
+           * anything: the gate above means no bundle exists yet.
+           */
           BLUESKY_PROXY_HEADER.set(header)
-          agent.configureProxy(header as any)
           hasConfiguredProxy = true
           setIsProxyConfigured(true)
         }}