🛡️ Sentinel: [CRITICAL] Fix command injection in id-lookups

[Donach]

Severity: CRITICAL
Vulnerability: A potential command injection vulnerability existed in packages/core/src/unix/id-lookups.ts where dynamic variables (groupName, username) were passed to execSync via string interpolation. An attacker could exploit this by passing unvalidated inputs to execute arbitrary shell commands.
Impact: Arbitrary shell command execution on the host system.
Fix: Refactored getGidFromGroupName, getUidFromUsername, and getHomedirFromUsername to use execFileSync, which spawns the executable directly without an intermediate shell. Passed inputs safely as an array of arguments, using the -- delimiter to prevent arguments starting with a dash from being parsed as options.
Verification: Ran pnpm lint, pnpm test, and pnpm build to verify the refactoring logic and ensure that no regressions were introduced.

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PR created automatically by Jules for task 9318451421827350139 started by @Donach

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