🛡️ Sentinel: [HIGH] Fix command injection in toolExists

[acebytes]

🚨 Severity: HIGH
💡 Vulnerability: Found a command injection vulnerability in toolExists within Sources/Cacheout/Models/CacheCategory.swift where user-controlled input (tool) was directly interpolated into a shell wrapper: shell("/usr/bin/which \(tool)").
🎯 Impact: If requiresTool dynamically passed to toolExists contained any shell metacharacters or spaces, it could execute arbitrary code, compromising the application process.
🔧 Fix: Refactored toolExists to eliminate the shell execution entirely. It now explicitly spawns a Process calling /usr/bin/env with ["which", tool] passed cleanly as process arguments, bypassing any shell parsing.
✅ Verification: Verified compilation and correctness. The toolExists check safely routes via the Process API avoiding shell evaluation.

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PR created automatically by Jules for task 10454670355606927541 started by @acebytes

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