🛡️ Sentinel: [HIGH] Fix command injection in user-manager.ts

[Donach]

🚨 Severity: HIGH
💡 Vulnerability: Command injection vulnerability identified in @agor/core/unix/user-manager.ts where the unixUserExists function passed a string-interpolated username directly into an execSync shell command (id "${username}"). This allowed arbitrary command execution if the input username broke out of the quoting.
🎯 Impact: High risk of local privilege escalation or arbitrary system command execution as the node process user, assuming an attacker was able to pass malformed input strings into contexts that relied on the check.
🔧 Fix: Upgraded the shell command from execSync to execFileSync, breaking out the command components into an argument array and neutralizing shell interpretation entirely. Added a -- option boundary before the dynamic username argument to further prevent option injection risks.
✅ Verification: Tested via unit tests and verified the fix mitigates injection payloads (e.g. $(id) or "; cat /etc/passwd #) by causing them to be interpreted as literal string targets by execFileSync.

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PR created automatically by Jules for task 13176355488770085314 started by @Donach

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