repoint the last AtpAgent statics and drop the dead conversions

Five sites read `AtpAgent.appLabelers`, which the previous commit stopped
populating - a silent regression, since an empty static reads as "no
authorities" rather than failing. All now read `Client.appLabelers`, the static
that is actually configured:

- `lib/moderation.ts`'s `isAppLabeler`, which decides whether a labeler is
  non-removable in the UI;
- `preferences/moderation.ts`'s `useMyLabelersQuery`, which seeds the labeler
  list with the app authorities;
- `feed/custom.ts`'s hand-rolled logged-out `getFeed`, which reproduces the
  header lex would have emitted because the request bypasses the client.

`sessionAccountToSession` (the `AtpSessionData` conversion) had no callers left
once the bridge agent went, and the age assurance config read - the one
standalone `new AtpAgent` the age assurance slice deliberately left behind -
moves to the public appview client, which is what it was approximating.

That is the last `AtpAgent` in src. What remains of `@atproto/api` in the
session layer is types and `BSKY_LABELER_DID`, which S4d owns.

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

src/lib/moderation.ts

@@ -1,5 +1,7 @@
 import {useMemo} from 'react'
-import {AtpAgent, type ComAtprotoLabelDefs} from '@atproto/api'
+import {type ComAtprotoLabelDefs} from '@atproto/api'
+import {Client} from '@atproto/lex'
+import {type DidString} from '@atproto/syntax'
 import {
   type InterpretedLabelValueDefinition,
   LABELS,
@@ -105,9 +107,9 @@ export function isAppLabeler(
     | app.bsky.labeler.defs.LabelerViewDetailed,
 ): boolean {
   if (typeof labeler === 'string') {
-    return AtpAgent.appLabelers.includes(labeler)
+    return Client.appLabelers.includes(labeler as DidString)
   }
-  return AtpAgent.appLabelers.includes(labeler.creator.did)
+  return Client.appLabelers.includes(labeler.creator.did)
 }
 
 export function isLabelerSubscribed(