🛡️ Sentinel: [HIGH] 숨겨진 파일 노출(Sensitive Data Exposure) 취약점 수정

[seonghobae]

🚨 Severity: HIGH
💡 Vulnerability: 숨겨진 파일 및 디렉토리(.git, .env 등)가 HTML 인덱스에 포함되어 민감한 정보가 노출될 위험이 존재했습니다.
🎯 Impact: 공격자가 저장소의 히스토리나 환경 설정 파일을 획득하여 심각한 보안 침해를 유발할 수 있습니다.
🔧 Fix: src/main/kotlin/html4tree/main.kt에서 디렉토리 순회 및 인덱스 생성 시 .으로 시작하는 숨겨진 항목을 무시하도록 필터(!it.name.startsWith("."))를 추가했습니다.
✅ Verification: src/test/kotlin/html4tree/MainTest.kt에 숨겨진 파일과 디렉토리가 출력 및 순회되지 않음을 검증하는 단위 테스트(testGoIgnoresHiddenFilesAndDirectories)를 추가하고 100% 테스트 커버리지를 달성했습니다.

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PR created automatically by Jules for task 10966133501192312716 started by @seonghobae

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