refactor(tools): add agent category for Task tool

Add visual distinction for the Task tool (subagent invocation) by
introducing a new 'agent' category with purple border styling. This
separates subagent tasks from regular task management tools
(TaskCreate, TaskUpdate, etc.) for clearer user feedback.

Also refactor terminal command layout in OneLineDisplay to properly
nest flex containers, fixing copy button alignment issues.

src/components/chat/tools/ToolRenderer.tsx

@@ -29,6 +29,7 @@ function getToolCategory(toolName: string): string {
   if (toolName === 'Bash') return 'bash';
   if (['TodoWrite', 'TodoRead'].includes(toolName)) return 'todo';
   if (['TaskCreate', 'TaskUpdate', 'TaskList', 'TaskGet'].includes(toolName)) return 'task';
+  if (toolName === 'Task') return 'agent';  // Subagent task
   if (toolName === 'exit_plan_mode' || toolName === 'ExitPlanMode') return 'plan';
   return 'default';
 }