migrate link resolution to the appview and chat clients

`resolveLink` took an agent and used it for four appview reads plus the chat
invite preview, so it now takes both clients as a `LinkResolvers` pair - the
caller cannot know which branch a URL will take until it is parsed. That was
the last DM_SERVICE_HEADERS site, so the constant is deleted.

`resolveGif` never touched the agent at all (it is pure URL metadata work on
what the picker already returned), so its parameter is dropped rather than
replaced, along with the one on `fetchResolveGifQuery`.

With the resolvers on clients, `apilib.post` loses the agent parameter the
previous slice kept solely for them, and `composerStateToDraft` takes the
resolver pair instead of minting a throwaway public agent.

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

src/view/com/composer/Composer.tsx

@@ -97,8 +97,8 @@ import {usePreferencesQuery} from '#/state/queries/preferences'
 import {useProfileQuery} from '#/state/queries/profile'
 import {resolveLinkQueryOptions} from '#/state/queries/resolve-link'
 import {
-  useAgent,
   useAppviewClient,
+  useChatClient,
   usePdsClient,
   useSession,
 } from '#/state/session'
@@ -280,8 +280,8 @@ export const ComposePost = ({
   const videoMaxDurationMs = allow10MinuteVideos
     ? VIDEO_10_MINUTE_MAX_DURATION_MS
     : VIDEO_MAX_DURATION_MS
-  const agent = useAgent()
   const client = useAppviewClient()
+  const chatClient = useChatClient()
   const pdsClient = usePdsClient()
   const queryClient = useQueryClient()
   const currentDid = currentAccount!.did
@@ -1003,7 +1003,7 @@ export const ComposePost = ({
     .map(post => post.embed.link!.uri)
   const linkQueries = useQueries({
     queries: linkUris.map(uri => ({
-      ...resolveLinkQueryOptions(agent, uri),
+      ...resolveLinkQueryOptions({appviewClient: client, chatClient}, uri),
       enabled: false,
     })),
   })
@@ -1094,12 +1094,13 @@ export const ComposePost = ({
     try {
       logger.info(`composer: posting...`)
       postUri = (
-        await apilib.post(agent, queryClient, {
+        await apilib.post(queryClient, {
           thread: filteredThread,
           replyTo: replyTo?.uri,
           onStateChange: setPublishingStage,
           langs: currentLanguages,
           appviewClient: client,
+          chatClient,
           pdsClient,
         })
       ).uris[0]
@@ -1294,8 +1295,8 @@ export const ComposePost = ({
   }, [
     l,
     ax,
-    agent,
     client,
+    chatClient,
     pdsClient,
     canPost,
     isPublishing,