[Security] Add command safety check to execCommand

[gonzaloriestra]

WHY are these changes introduced?

execCommand was missing a security check that other command execution utilities in @shopify/cli-kit had. Specifically, it didn't verify if the binary was located in the current working directory, which could lead to PATH hijacking vulnerabilities.

WHAT is this pull request doing?

• Modifies execCommand in packages/cli-kit/src/public/node/system.ts to call checkCommandSafety.
• Uses parseCommand to extract the binary name from the command string.
• Adds a unit test in packages/cli-kit/src/public/node/system.test.ts to verify the fix.
• Adds security-focused documentation to checkCommandSafety.

How to test your changes?

Run the unit tests for cli-kit:
pnpm --filter @shopify/cli-kit vitest run src/public/node/system.test.ts

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 8108638445474048430 started by @gonzaloriestra

[google-labs-jules]

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