🛡️ Sentinel: [CRITICAL] Fix prefix-matching path traversal bypass in Lua IO

[mleem97]

🚨 Severity: CRITICAL
💡 Vulnerability: The ResolveSafe function in GregIoLuaModule.cs used StartsWith on directory paths without ensuring a trailing slash. This allowed a Lua mod with a data directory mod/data to access sibling directories like mod/data_secret because "mod/data_secret".StartsWith("mod/data") evaluates to true.
🎯 Impact: Malicious Lua mods could break out of their sandboxed data directory and read/write files in sibling directories, potentially exposing sensitive data or other mods' private states.
🔧 Fix: Appended a directory separator to the base path before the StartsWith check, and added an exact match check for the base directory itself. Also documented the prefix-matching bypass in .jules/sentinel.md.
✅ Verification: Ran dotnet test tests/gregCore.Tests.csproj and manually verified path parsing logic during patching.

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PR created automatically by Jules for task 5407275354046974236 started by @mleem97

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