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

[acebytes]

This PR introduces a defense-in-depth security enhancement by refactoring the toolExists method in CacheCategory.swift. It replaces the unsafe string interpolation inside a shell wrapper (/usr/bin/which \(tool)) with direct Process execution, passing the tool name safely via the arguments array. This prevents potential command injection risks if the tool argument ever becomes user-controlled.

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PR created automatically by Jules for task 10678069811023322393 started by @acebytes

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