🛡️ Sentinel: [CRITICAL] Fix path traversal in CustomEmployeeManager

[mleem97]

🚨 Severity: CRITICAL
💡 Vulnerability: The id parameter in CustomEmployeeManager.Register is not validated, which allows path traversal attacks when the id is subsequently used to construct file paths for the employee's portrait image in SetPortrait.
🎯 Impact: An attacker or malicious mod could register an employee with an ID like ../../secrets.txt leading to arbitrary file read issues when the mod attempts to load portrait assets.
🔧 Fix: Added input validation for the id argument in the CustomEmployeeManager.Register method to check for invalid filename characters (Path.GetInvalidFileNameChars()) and ... If found, registration is rejected.
✅ Verification: Verified that tests pass via dotnet test and the code builds successfully. The added journal entry in .jules/sentinel.md documents this critical learning.

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PR created automatically by Jules for task 11384705067694518917 started by @mleem97

[google-labs-jules]

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