feat(header): session tabs are opt-in

Default off; the changelog entry points at Settings → General →
Navigation → Session tabs and mentions the Alt+W close shortcut.

packages/ui/src/stores/useUIStore.ts

@@ -750,7 +750,7 @@ interface UIStore {
   maxLastMessageLength: number; // chars — truncate {last_message} when summarization is off
 
   showTerminalQuickKeysOnDesktop: boolean;
-  /** Header session tabs (web/desktop). Off restores the plain session title. */
+  /** Header session tabs (web/desktop), opt-in. Off keeps the plain session title. */
   sessionTabsEnabled: boolean;
   persistChatDraft: boolean;
   showOpenCodeUpdateNotifications: boolean;
@@ -1093,7 +1093,7 @@ export const useUIStore = create<UIStore>()(
         maxLastMessageLength: 250,
 
         showTerminalQuickKeysOnDesktop: false,
-        sessionTabsEnabled: true,
+        sessionTabsEnabled: false,
         persistChatDraft: true,
         showOpenCodeUpdateNotifications: !isWindowsArm64(),
         agentControlToolEnabled: true,