fix(ui): show embedded subagent history while visibility stays inactive

Busy context-panel session chats could render only the working-status row
when the iframe booted inactive or lost its visibility handshake, because
message reads shared the composer/background-work gate. Keep message
subscriptions enabled in the mounted session-chat panel so materialized
history remains visible (#2903, #2892).

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/App.tsx

@@ -209,6 +209,11 @@ const EmbeddedSessionChatContent: React.FC<{
       <OpenCodeUpdateToast />
       <ChatView
         active={embeddedBackgroundWorkEnabled}
+        // Always subscribe to message history in the mounted session-chat
+        // iframe. Visibility still gates composer focus and background work so
+        // a boot-inactive / lost-handshake race cannot leave a busy subagent
+        // showing only its status row (#2903 / #2892).
+        messagesEnabled={true}
         readOnly={embeddedSessionChat.readOnly}
         initialAllowPromptingSubagentSessions={embeddedSessionChat.allowPromptingSubagentSessions}
       />