feat: replace task lane X with PanelRightClose icon

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

frontend/src/components/assistant/TaskLane.tsx

@@ -1,7 +1,7 @@
 import { useState, useEffect, useRef, useCallback } from 'react'
 import {
   Copy, Check, SkipForward, Terminal, ChevronDown, ChevronUp,
-  Send, Clipboard, Loader2, X, MessageCircleQuestion, Eye,
+  Send, Clipboard, Loader2, PanelRightClose, MessageCircleQuestion, Eye,
 } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'
@@ -247,8 +247,8 @@ export function TaskLane({ questions, actions, sessionId, onSubmit, onClose, loa
             {allHandled ? '✓ Ready' : `${doneCount}/${totalCount}`}
           </span>
         </h3>
-        <button onClick={onClose} className="text-muted-foreground hover:text-heading transition-colors p-1">
-          <X size={16} />
+        <button onClick={onClose} className="text-muted-foreground hover:text-heading transition-colors p-1" title="Collapse tasks">
+          <PanelRightClose size={16} />
         </button>
       </div>