🛡️ Sentinel: [HIGH] Fix command injection in tool discovery

[acebytes]

🚨 Severity: HIGH
💡 Vulnerability: Command injection vulnerability via string interpolation in shell wrapper function toolExists.
🎯 Impact: Arbitrary command execution if a malicious tool name is probed.
🔧 Fix: Replaced shell("/usr/bin/which \(tool)") with a direct Foundation.Process execution using explicitly defined arguments (process.arguments = ["which", tool]).
✅ Verification: Code statically verified to ensure no string interpolation is used for shell commands. Verified tests via ./scripts/test.sh.

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PR created automatically by Jules for task 13979116635724391514 started by @acebytes

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