feat: task lane close preserves state instead of clearing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/AssistantChatPage.tsx

@@ -801,7 +801,6 @@ export default function AssistantChatPage() {
           onSubmit={handleTaskSubmit}
           onClose={() => {
             setShowTaskLane(false)
-            if (activeChatId) clearTaskState(activeChatId)
           }}
           loading={loading}
         />