🛡️ Sentinel: [CRITICAL] Fix path traversal bypass in Lua IO module

[mleem97]

🚨 Severity: CRITICAL
💡 Vulnerability: A prefix-matching path traversal bypass existed in GregIoLuaModule.cs. The module used String.StartsWith to ensure the resolved path started with the sandbox base directory. However, because the base directory string did not strictly end in a directory separator, a requested path like /base/data-secret would successfully evaluate as StartsWith("/base/data") allowing arbitrary read/write access to matching folders outside the intended sandbox.
🎯 Impact: Malicious Lua scripts could potentially escape the intended sandbox (/modDir/data/) and read, write, or delete files in adjacent directories that share the same string prefix, potentially compromising mod or game data.
🔧 Fix: Appended Path.DirectorySeparatorChar to dataDirFull if it doesn't already have a trailing separator before performing the StartsWith check. Documented the pattern in .jules/sentinel.md.
✅ Verification: Compiled the solution successfully (dotnet build gregCore.csproj -c Release) to ensure no regressions and verified logic locally.

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PR created automatically by Jules for task 7332935971261093362 started by @mleem97

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