🛡️ Sentinel: [CRITICAL] Fix TOCTOU vulnerability in cleanup logging

[acebytes]

🚨 Severity: CRITICAL
💡 Vulnerability: FileHandle(forWritingTo:) and String.write(to:atomically:) follow symlinks by default. This makes the code vulnerable to a Time-of-Check to Time-of-Use (TOCTOU) symlink attack when appending to the cleanup log file.
🎯 Impact: An attacker could replace the ~/.cacheout/cleanup.log file with a symlink pointing to a sensitive file. The application would then write to that sensitive file, potentially escalating privileges or causing data loss.
🔧 Fix: Replaced the high-level file APIs with a secure POSIX open() call using O_CREAT | O_WRONLY | O_APPEND | O_NOFOLLOW | O_CLOEXEC flags to explicitly refuse symlinks, then wrapped the resulting file descriptor in a FileHandle.
✅ Verification: Ensure the .cacheout/cleanup.log is written successfully without following symlinks.

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PR created automatically by Jules for task 132128948724264125 started by @acebytes

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