[Refactor] Use constant lookup map for architectures in platformAndArch

[gonzaloriestra]

WHY are these changes introduced?

This clean-up simplifies and modularizes the CPU architecture determination in platformAndArch (part of the @shopify/cli-kit workspace).

WHAT is this pull request doing?

Refactors platformAndArch in packages/cli-kit/src/public/node/os.ts by:

1. Extracting the mapping of x64 to amd64 and ia32 to 386 into a clean, top-level lookup constant (ARCH_MAP).
2. Eliminating the imperative if/else block and using nullish coalescing (??) to fallback to the raw architecture string.

No observable behavior changes are introduced.

How to test your changes?

CI

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 for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

---

PR created automatically by Jules for task 9085943036394304505 started by @gonzaloriestra

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.