feat: add exam skill for CS question generation

- SKILL.md: agent instructions for generating exam questions
- validate.py: Python schema validation (jsonschema with fallback)
- validate.mjs: Node.js schema validation (built-in modules only)

Supports7 question types: fill_blank, single_choice, multiple_choice,
true_false, short_answer, code_reading, scenario.

exam/.claude/skills/exam/SKILL.md

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+---
+name: exam
+description: "Generate, validate, and manage CS exam questions. Trigger: '生成题目', 'generate questions', 'validate', '添加题目', '/exam'"
+---
+
+# CS 知识应试强化 — 题目生成 Skill
+
+为 `topics/` 目录生成符合 JSON Schema 的题目文件。
+
+## 目录结构
+
+```
+examination/
+├── schema/
+│   ├── question.schema.json    # 题目 JSON Schema (Draft-07)
+│   ├── prompt-template.md      # 提示词模板参考
+│   └── templates/              # 各题型示例
+├── topics/
+│   ├── index.json              # 主题索引
+│   └── {group}/{subtopic}/
+│       ├── meta.json           # 子主题元信息
+│       ├── fill_blank.json     # 填空题
+│       └── single_choice.json  # 单选题
+└── .claude/skills/exam/
+    ├── SKILL.md                # 本文件
+    ├── validate.py             # Python 校验脚本
+    └── validate.mjs            # Node.js 校验脚本
+```
+
+## 工作流
+
+### 1. 确定目标
+
+用户会说类似「生成填空题」「给 gc-jvm 加10道选择题」。你需要：
+
+1. 确认 **子主题**：读 `topics/index.json` 找到对应 slug 和 path
+2. 确认 **题型**：fill_blank / single_choice / multiple_choice / true_false / short_answer / code_reading / scenario
+3. 确认 **数量**：默认 10
+
+如果用户没指定，主动询问。
+
+### 2. 读取上下文
+
+```bash
+# 查看子主题已有题目，避免重复
+cat topics/{group}/{subtopic}/meta.json
+cat topics/{group}/{subtopic}/{type}.json  # 如果存在
+```
+
+### 3. 生成题目
+
+你（Claude）直接生成 JSON。遵循以下规则：
+
+- 输出格式严格匹配 `schema/question.schema.json`
+- ID 格式：`{type_short}-{seq}`，如 `fb-001`，序号从已有最大值 +1 开始
+- type_short 映射：sc=single_choice, mc=multiple_choice, tf=true_false, fb=fill_blank, sa=short_answer, cr=code_reading, sn=scenario
+- difficulty 1-5，根据主题上下文合理分布
+- tags 使用子主题 meta.json 中的 tags 作为参考
+- explanation 必须详细，解释为什么对/错
+
+生成后，将 JSON 写入临时文件进行校验：
+
+```bash
+# 写入临时文件
+cat > /tmp/exam_gen.json << 'ENDJSON'
+{ ...生成的JSON... }
+ENDJSON
+```
+
+### 4. 校验
+
+根据可用运行时选择其一：
+
+```bash
+# Python (优先)
+python3 .claude/skills/exam/validate.py /tmp/exam_gen.json
+
+# Node.js (备选)
+node .claude/skills/exam/validate.mjs /tmp/exam_gen.json
+```
+
+- 校验通过 → 继续写入
+- 校验失败 → 修正 JSON 后重新校验，直到通过
+
+### 5. 写入文件
+
+校验通过后：
+
+1. 如果目标题型文件已存在，读取现有 questions 数组，追加新题目（不覆盖旧题）
+2. 如果不存在，创建新文件
+3. 更新 `meta.json` 的 `question_files` 和 `stats`
+4. 更新 `topics/index.json` 中对应子主题的 `stats`
+
+文件格式：
+
+```json
+{
+  "topic": "{subtopic-slug}",
+  "type": "{question_type}",
+  "schema_version": "1.0.0",
+  "generated": "{ISO-8601}",
+  "questions": [ ... ]
+}
+```
+
+### 6. 确认
+
+报告生成结果：题型、数量、文件路径。
+
+提示用户在线查看：
+
+> ✅ 已完成！前往 http://47.121.181.112:30000/ 查看新题目。
+
+## 校验脚本
+
+两个脚本功能相同，检测可用环境后选用：
+
+- `.claude/skills/exam/validate.py` — 优先用 `jsonschema`，不可用时回退到基础校验
+- `.claude/skills/exam/validate.mjs` — 纯 Node.js 内置模块，无外部依赖
+
+退出码：0=通过，1=有错误，2=用法错误。
+
+## 7 种题型速查
+
+| 题型 | type | 关键字段 |
+|------|------|----------|
+| 填空 | fill_blank | answer: string[], answer_rule: "any"/"all"/"ordered" |
+| 单选 | single_choice | options: {A-D}, answer: string |
+| 多选 | multiple_choice | options: {A-D}, answer: string[] |
+| 判断 | true_false | answer: boolean |
+| 简答 | short_answer | answer: string, keywords: string[], scoring_rubric: string |
+| 代码阅读 | code_reading | code, language, sub_questions[] |
+| 场景分析 | scenario | context, sub_questions[] |

exam/.claude/skills/exam/validate.mjs

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+#!/usr/bin/env node
+/**
+ * Validate a question JSON file against schema/question.schema.json.
+ *
+ * Usage:
+ *   node validate.mjs <json_file>
+ *   echo '<json>' | node validate.mjs -
+ *
+ * Exit codes: 0 = valid, 1 = errors found, 2 = usage error.
+ */
+
+import { readFileSync } from "node:fs";
+import { resolve, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+import { createRequire } from "node:module";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = resolve(__dirname, "../../..");
+const SCHEMA_PATH = resolve(REPO_ROOT, "schema/question.schema.json");
+
+function loadJson(path) {
+  const raw = path === "-" ? readFileSync(0, "utf-8") : readFileSync(path, "utf-8");
+  return JSON.parse(raw);
+}
+
+const VALID_TYPES = [
+  "single_choice", "multiple_choice", "true_false",
+  "fill_blank", "short_answer", "code_reading", "scenario",
+];
+
+const ID_RE = /^(sc|mc|tf|fb|sa|cr|sn)-\d{3}$/;
+
+function validate(data) {
+  const errors = [];
+  const requiredTop = ["topic", "type", "schema_version", "questions"];
+  for (const field of requiredTop) {
+    if (!(field in data)) errors.push(`Missing required field: ${field}`);
+  }
+  if (!VALID_TYPES.includes(data.type)) errors.push(`Invalid type: ${data.type}`);
+  if (data.schema_version !== "1.0.0")
+    errors.push(`schema_version must be '1.0.0', got '${data.schema_version}'`);
+
+  for (const [i, q] of (data.questions || []).entries()) {
+    const p = `questions[${i}]`;
+    for (const field of ["id", "type", "difficulty", "tags", "question", "explanation"]) {
+      if (!(field in q)) errors.push(`${p}: missing '${field}'`);
+    }
+    if (!VALID_TYPES.includes(q.type)) errors.push(`${p}: invalid type '${q.type}'`);
+    if (q.difficulty !== undefined && !(q.difficulty >= 1 && q.difficulty <= 5))
+      errors.push(`${p}: difficulty must be 1-5`);
+    if (q.id !== undefined && !ID_RE.test(q.id))
+      errors.push(`${p}: id '${q.id}' doesn't match pattern {type}-{000}`);
+  }
+  return errors;
+}
+
+function main() {
+  if (process.argv.length !== 3) {
+    console.error(`Usage: node ${process.argv[1]} <json_file | ->`);
+    process.exit(2);
+  }
+  const data = loadJson(process.argv[2]);
+  const errors = validate(data);
+
+  if (errors.length) {
+    console.log(`❌ ${errors.length} error(s):`);
+    for (const e of errors) console.log(`  • ${e}`);
+    process.exit(1);
+  } else {
+    console.log(`✅ Valid — ${(data.questions || []).length} question(s)`);
+    process.exit(0);
+  }
+}
+
+main();

exam/.claude/skills/exam/validate.py

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+#!/usr/bin/env python3
+"""Validate a question JSON file against schema/question.schema.json.
+
+Usage:
+    python3 validate.py <json_file>
+    echo '<json>' | python3 validate.py -
+
+Exit codes: 0 = valid, 1 = errors found, 2 = usage error.
+"""
+import json
+import sys
+from pathlib import Path
+
+# Resolve schema path relative to repo root (3 levels up from this script)
+SCRIPT_DIR = Path(__file__).resolve().parent
+REPO_ROOT = SCRIPT_DIR.parent.parent.parent
+SCHEMA_PATH = REPO_ROOT / "schema" / "question.schema.json"
+
+
+def load_json(path: str) -> dict:
+    if path == "-":
+        return json.load(sys.stdin)
+    with open(path, encoding="utf-8") as f:
+        return json.load(f)
+
+
+def validate(data: dict, schema: dict) -> list[str]:
+    """Validate data against schema. Returns list of error strings."""
+    errors = []
+    try:
+        from jsonschema import Draft7Validator
+        validator = Draft7Validator(schema)
+        for err in sorted(validator.iter_errors(data), key=lambda e: list(e.path)):
+            path = ".".join(str(p) for p in err.absolute_path) or "(root)"
+            errors.append(f"{path}: {err.message}")
+        return errors
+    except ImportError:
+        pass
+
+    # Fallback: basic manual validation
+    required_top = ["topic", "type", "schema_version", "questions"]
+    for field in required_top:
+        if field not in data:
+            errors.append(f"Missing required field: {field}")
+
+    valid_types = [
+        "single_choice", "multiple_choice", "true_false",
+        "fill_blank", "short_answer", "code_reading", "scenario",
+    ]
+    if data.get("type") not in valid_types:
+        errors.append(f"Invalid type: {data.get('type')}")
+
+    if data.get("schema_version") != "1.0.0":
+        errors.append(f"schema_version must be '1.0.0', got '{data.get('schema_version')}'")
+
+    for i, q in enumerate(data.get("questions", [])):
+        prefix = f"questions[{i}]"
+        for field in ["id", "type", "difficulty", "tags", "question", "explanation"]:
+            if field not in q:
+                errors.append(f"{prefix}: missing '{field}'")
+        if q.get("type") not in valid_types:
+            errors.append(f"{prefix}: invalid type '{q.get('type')}'")
+        if "difficulty" in q and not (1 <= q["difficulty"] <= 5):
+            errors.append(f"{prefix}: difficulty must be 1-5")
+        if "id" in q:
+            import re
+            if not re.match(r"^(sc|mc|tf|fb|sa|cr|sn)-\d{3}$", q["id"]):
+                errors.append(f"{prefix}: id '{q['id']}' doesn't match pattern {{type}}-{{000}}")
+
+    return errors
+
+
+def main():
+    if len(sys.argv) != 2:
+        print(f"Usage: {sys.argv[0]} <json_file | ->", file=sys.stderr)
+        sys.exit(2)
+
+    schema = load_json(str(SCHEMA_PATH))
+    data = load_json(sys.argv[1])
+    errors = validate(data, schema)
+
+    if errors:
+        print(f"❌ {len(errors)} error(s):")
+        for e in errors:
+            print(f"  • {e}")
+        sys.exit(1)
+    else:
+        print(f"✅ Valid — {len(data.get('questions', []))} question(s)")
+        sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()