🛡️ Sentinel: [CRITICAL] Fix command injection vulnerability

[raccioly]

🚨 Severity: CRITICAL
💡 Vulnerability: Found multiple instances across the CLI tools where execSync was used, which allows shell interpolation and makes the application vulnerable to command injection if dynamic or user-controlled inputs are passed (e.g., the name argument in isCliAvailable or inputs parsed directly into Git commands).
🎯 Impact: Attackers could execute arbitrary shell commands via command injection if an input string is not thoroughly sanitized and contains shell metacharacters like ;, &&, or ||.
🔧 Fix: Replaced execSync with execFileSync in setup.mjs, score.mjs, freshness.mjs, ensure-skills.mjs, and shared-git.mjs. Adjusted commands that rely on piping (| wc -l) or redirection (2>/dev/null) to use native JavaScript counting and child process configuration parameters (stdio: ['pipe', 'pipe', 'ignore']).
✅ Verification: Ran the full automated test suite to ensure that command execution behavior operates correctly natively without regressions.

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PR created automatically by Jules for task 515927768858953820 started by @raccioly

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