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

[Donach]

🚨 Severity: CRITICAL
💡 Vulnerability: Unsanitized user inputs (username, groupName, sudoUser) were concatenated into shell strings via execSync(), leaving the system highly vulnerable to command injection.
🎯 Impact: An attacker could craft a malicious username or group name containing shell metacharacters (e.g., ; rm -rf /) that would be executed with the privileges of the Node.js application (often root when running daemon sync operations).
🔧 Fix: Migrated all execSync usages handling dynamic input to execFileSync, passing user input directly via an argument array to completely bypass the shell's command interpolation, nullifying the command injection vector.
✅ Verification: Verified by ensuring the code passes pnpm lint and pnpm test. The CI workflow should succeed cleanly.

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PR created automatically by Jules for task 1592603489307158644 started by @Donach

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[dhavli]

Closing — overlaps with #45 which is more comprehensive (includes test updates). Note: sync-unix.ts fix from this PR is not in #45 — should be a separate PR.