🛡️ Sentinel: [CRITICAL] Fix command injection in ID lookups

[Donach]

🚨 Severity: CRITICAL
💡 Vulnerability: Shell command injection vulnerability in packages/core/src/unix/id-lookups.ts. User-controlled inputs (username and groupName) were passed to execSync via string interpolation, allowing an attacker to inject arbitrary shell commands.
🎯 Impact: Remote code execution (RCE) with the privileges of the daemon process.
🔧 Fix:

• Replaced execSync with execFileSync to execute commands directly without a shell interpreter.
• Passed user inputs as arguments in an array instead of concatenating strings.
• Used the POSIX -- delimiter before user inputs (e.g., ['passwd', '--', username]) to strictly separate options from arguments, protecting against flag/argument injection.
• Updated .jules/sentinel.md journal with the learning to use execFileSync over execSync for dynamic arguments.
  ✅ Verification:
• pnpm -w run lint:fix succeeds with no formatting issues.
• pnpm --filter @agor/core test -- src/unix passes all unit tests for the core package.

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PR created automatically by Jules for task 8323551273461768723 started by @Donach

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