🛡️ Sentinel: [HIGH] Fix unsafe permission handling

[acebytes]

🚨 Severity: HIGH\n💡 Vulnerability: POSIX file operations were not verifying return codes, leading to fail-open behavior.\n🎯 Impact: Security hardening constraints could be bypassed, leaving sensitive files with weak permissions.\n🔧 Fix: Added explicit return code validation to fail securely.\n✅ Verification: Verified test suite and manual verification.

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PR created automatically by Jules for task 3447694017941032563 started by @acebytes

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