🛡️ Sentinel: [HIGH] Fix path traversal in scene downloads

[ImChong]

• 🚨 Severity: HIGH
• 💡 Vulnerability: 路径穿越 (Path Traversal / Zip Slip)。downloadExampleScenesFolder 方法从 JSON 获取的相对路径未经过滤即写入虚拟机文件系统，攻击者可能通过构造恶意的 files.json 将文件写入 /working/ 之外的任意目录。
• 🎯 Impact: 攻击者可修改 MuJoCo 虚拟环境内的系统配置或重写其他加载的组件，导致沙盒内代码执行环境被篡改。
• 🔧 Fix: 在写入前，增加针对 /(^|\/)\.\.(\/|$)/ 结构和 / 开头的校验逻辑，检测到越权路径时抛出异常。
• ✅ Verification: 运行测试并验证了正则不会误杀类似 my..file.txt 的合法文件名，仅过滤目录切换符。

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PR created automatically by Jules for task 10545686341381630955 started by @ImChong

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