🛡️ Sentinel: [CRITICAL] Fix command injection in Unix utilities

[Donach]

🚨 Severity: CRITICAL
💡 Vulnerability: Command injection via shell execution. execSync was being used with string interpolation for dynamic variables like username and groupName in id-lookups.ts and user-manager.ts.
🎯 Impact: If an attacker could control the username or group name input, they could inject shell metacharacters and execute arbitrary system commands as the daemon process.
🔧 Fix: Replaced execSync with execFileSync to bypass shell execution completely. Arguments are now passed securely as an array, explicitly cast to strings, and prefixed with -- to prevent argument injection. Shell redirection (> /dev/null) was replaced with { stdio: 'ignore' }. Updated corresponding test mocks.
✅ Verification: Ran pnpm build, pnpm lint:fix, and pnpm test for the @agor/core/unix package to ensure full test suite passes.

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PR created automatically by Jules for task 14995128911294700721 started by @Donach

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