refactor(ui): 移动并复用 isSkillSelected 函数

- 将 isSkillSelected 函数从 PromptInput.tsx 移动到 SlashCommandMenu.tsx
- 在 PromptInput.tsx 中从 SlashCommandMenu 导入 isSkillSelected
- 简化代码结构，避免函数重复定义
- 统一技能选择判断逻辑以提高代码复用性

Author: hqwlkj

src/ui/components/SkillsDropdown/index.tsx

@@ -1,8 +1,8 @@
 import DropdownMenu from "../../DropdownMenu";
 import React, { useEffect, useState } from "react";
-import { isSkillSelected } from "../../PromptInput";
 import type { SkillInfo } from "../../../session";
 import { useInput } from "ink";
+import { isSkillSelected } from "../../SlashCommandMenu";
 
 const SkillsDropdown: React.FC<{
   open: boolean;

src/ui/index.ts

@@ -23,7 +23,6 @@ export {
   IMAGE_ATTACHMENT_CLEAR_HINT,
   formatImageAttachmentStatus,
   formatSelectedSkillsStatus,
-  isSkillSelected,
   addUniqueSkill,
   toggleSkillSelection,
   removeCurrentSlashToken,