fix(desktop): let channel members bypass mention agent gate (#965)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

desktop/src/features/messages/lib/useMentions.ts

@@ -209,7 +209,11 @@ export function useMentions(
       if (isArchivedDiscovery(pubkey)) {
         return;
       }
-      if (candidate.isAgent && !mentionableAgentPubkeys.has(pubkey)) {
+      if (
+        candidate.isAgent &&
+        !candidate.isMember &&
+        !mentionableAgentPubkeys.has(pubkey)
+      ) {
         return;
       }