🛡️ Sentinel: [CRITICAL] Fix command injection in toolExists

[acebytes]

🚨 Severity: CRITICAL
💡 Vulnerability: Command injection risk existed in CacheCategory.swift within toolExists where shell("/usr/bin/which \(tool)") directly interpolated the dynamic tool string into a /bin/bash -c executed command string.
🎯 Impact: If the tool variable were ever derived dynamically or allowed any user-controlled inputs, it could have resulted in arbitrary command execution on the host machine.
🔧 Fix: Replaced the shell wrapper execution with direct Foundation.Process invocation using /usr/bin/env and passing the tool exclusively through process.arguments = ["which", tool].
✅ Verification: Tested manually that the fix correctly returns whether the tool exists via standard termination status without introducing command injection paths.

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PR created automatically by Jules for task 3052558096566051674 started by @acebytes

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