docs: note setDefaultOpenAIKey in quickstart

[dkundel-openai]

Summary

• mention setDefaultOpenAIKey() and setTracingExportApiKey() in quickstart

Testing

• pnpm lint
• pnpm -r build-check (fails: Cannot find module '@openai/agents-extensions')
• pnpm build
• CI=1 pnpm test

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https://chatgpt.com/codex/tasks/task_i_6848ca9d087c833185aecfe2c72ac7b7

[changeset-bot]

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