🛡️ Sentinel: [CRITICAL] Fix command injection in Unix user lookups

[Donach]

🚨 Severity: CRITICAL
💡 Vulnerability: Command injection in getGidFromGroupName, getUidFromUsername, getHomedirFromUsername, and unixUserExists.
🎯 Impact: Attackers could potentially execute arbitrary shell commands if they can control the username or groupName parameters passed to these functions.
🔧 Fix: Replaced execSync with execFileSync to bypass shell execution, used argument arrays, and utilized the -- double-dash to prevent argument injection.
✅ Verification: Ran pnpm -w run lint:fix and pnpm --filter @agor/core test. Tests pass and no new issues are introduced.

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PR created automatically by Jules for task 8025371133038674867 started by @Donach

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