[Refactor] Declarative createGitIgnore content construction

[gonzaloriestra]

WHY are these changes introduced?

This is part of automated task cleanups to improve codebase readability and maintainability.

WHAT is this pull request doing?

Refactors createGitIgnore in packages/cli-kit/src/public/node/git.ts to use a declarative array mapping .map().join('') rather than an imperative for...of loop with string concatenation.

How to test your changes?

CI

Post-release steps

None.

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 11382995813698359031 started by @gonzaloriestra

[google-labs-jules]

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