[Refactor] Use constant lookup map in platformAndArch

[gonzaloriestra]

WHY are these changes introduced?

To improve code readability, maintainability, and idiom compliance in packages/cli-kit/src/public/node/os.ts.

WHAT is this pull request doing?

Refactors platformAndArch to replace the imperative conditional if-else chain with a declarative ARCH_MAP lookup map and nullish coalescing fallback.

How to test your changes?

CI

Post-release steps

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

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PR created automatically by Jules for task 17466121781401712051 started by @gonzaloriestra

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