🛡️ Sentinel: [security improvement] Refactor toolExists to use direct Process execution

[acebytes]

This PR provides a defense-in-depth security enhancement to the CacheCategory.toolExists method. The previous implementation evaluated shell commands using string interpolation (shell("/usr/bin/which \(tool)")). Even though the inputs are statically defined, evaluating dynamic inputs using /bin/bash -c is an unsafe pattern that risks command injection. The code has been refactored to use a direct Foundation.Process execution of /usr/bin/which, passing tool strictly as an element in the arguments array and returning successfully when terminationStatus == 0. We also mapped standardOutput and standardError to FileHandle.nullDevice to prevent resource leaks.

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PR created automatically by Jules for task 14105587751512567198 started by @acebytes

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