🔒 Fix command injection in editor diff tool verification

[TrueAlpha-spiral]

🎯 What: The vulnerability fixed
Fixed a command injection vulnerability in the commandExists function of packages/core/src/utils/editor.ts.

⚠️ Risk: The potential impact if left unfixed
An attacker could potentially execute arbitrary commands on the user's system by passing maliciously crafted strings as the cmd argument, exploiting the fact that execSync spawns a shell and interpolates the command directly into the shell string.

🛡️ Solution: How the fix addresses the vulnerability
The solution replaces execSync with execFileSync, which prevents the execution of a shell altogether. Arguments are now passed securely as an array, eliminating the risk of shell injection. For cross-platform support, the function now specifically checks process.platform === 'win32' and executes where.exe [cmd], otherwise it executes which [cmd] (since command -v is a shell built-in). Tests were updated accordingly.

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PR created automatically by Jules for task 12797687912229606170 started by @TrueAlpha-spiral

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