🛡️ Sentinel: [CRITICAL] Fix command injection vulnerabilities

[raccioly]

🚨 Severity: CRITICAL
💡 Vulnerability: Unsanitized variables inside shell execution calls using execSync. If a directory name or path string contains shell metacharacters (&, ;, ||), an attacker could easily inject arbitrary code and spawn unauthorized processes.
🎯 Impact: Arbitrary remote code execution (RCE) / command injection. By replacing the shell execution, we bypass the OS shell entirely and pass arguments safely and directly to the binaries.
🔧 Fix: Refactored vulnerable invocations across cli/commands/setup.mjs, cli/commands/score.mjs, cli/ensure-skills.mjs, cli/validators/freshness.mjs, and cli/shared-git.mjs to use execFileSync. Removed shell features like pipes (| wc -l) and redirections (2>/dev/null), replacing them natively with JavaScript string splitting and native stdio ignores (['pipe', 'pipe', 'ignore']). Cleaned up unused execSync imports from surrounding files.
✅ Verification: Ran test suite (node --test tests/*.test.mjs) to verify git integrations and CLI initializations operate identically without regressions.

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PR created automatically by Jules for task 18162553312546432142 started by @raccioly

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