🛡️ Sentinel: [CRITICAL] Fix TOCTOU vulnerability

[acebytes]

🚨 Severity: CRITICAL
💡 Vulnerability: Fixed insecure file creation permissions that temporarily exposed data with default permissions before restricting them.
🎯 Impact: Prevents a local privilege escalation or information disclosure race condition (TOCTOU) where a local attacker could read sensitive contents.
🔧 Fix: Modified file writing to atomically create the temporary file using open() with O_CREAT | O_EXCL and 0600 permissions.
✅ Verification: Tests run successfully, and code review verifies secure POSIX open calls are used.

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PR created automatically by Jules for task 2812012228935702094 started by @acebytes

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