feat: add difficulty field to all stages

- Add difficulty field to stage configurations (easy/medium/hard)
- Update sync script to support stage-level difficulty
- Difficulty distribution:
  - Easy: hello, mario-less, cash, me
  - Medium: mario-more, caesar, readability, scrabble, songs
  - Hard: credit

Author: william-yangbo

scripts/sync-to-api.js

@@ -119,6 +119,7 @@ async function main() {
         // v2: 新字段
         languageConfigs: languageConfigs,
         isFree: isFree,
+        difficulty: s.difficulty, // stage 级别的难度
         // 向后兼容: 保留主语言作为 language 字段
         language: primaryLanguage,
         // 从 languageConfigs 中提取主语言的配置（向后兼容）

stages/caesar.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: caesar
 name: "Caesar"
+difficulty: medium
 description: caesar.md
 summary: |
   实现凯撒密码加密程序，使用命令行参数指定密钥。

stages/cash.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: cash
 name: "Cash"
+difficulty: easy
 description: cash.md
 summary: |
   实现贪心算法，计算找零所需的最少硬币数量。

stages/credit.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: credit
 name: "Credit"
+difficulty: hard
 description: credit.md
 summary: |
   实现信用卡号验证程序，使用 Luhn 算法。

stages/hello.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: hello
 name: "Hello"
+difficulty: easy
 description: hello.md
 summary: |
   实现一个程序，向用户打招呼。

stages/mario-less.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: mario-less
 name: "Mario (Less)"
+difficulty: easy
 description: mario-less.md
 summary: |
   使用 # 打印半个金字塔，练习循环和条件语句。

stages/mario-more.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: mario-more
 name: "Mario (More)"
+difficulty: medium
 description: mario-more.md
 summary: |
   实现双侧金字塔版本的 Mario 程序。

stages/me.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: me
 name: "Me"
+difficulty: easy
 description: me.md
 summary: |
   实现一个程序，接收用户输入的名字并输出问候语。

stages/readability.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: readability
 name: "Readability"
+difficulty: medium
 description: readability.md
 summary: |
   实现一个程序来计算文本的可读性等级。

stages/scrabble.yml

@@ -3,6 +3,7 @@ version: "1.0"
 
 slug: scrabble
 name: "Scrabble"
+difficulty: medium
 description: scrabble.md
 summary: |
   实现一个程序来判断 Scrabble 游戏中哪位玩家获胜。