chore(docs): add Built with Fern shield to CLI README

[jon-fern]

Description

Adds the "Built with Fern" shield badge to the top of the CLI README, matching the badge used in generated SDK readmes.

Changes Made

• Added [![fern shield](https://img.shields.io/badge/...)](https://buildwithfern.com...) badge to the shield row in README.md

Testing

• Manual testing completed — verified badge renders correctly in markdown preview

Link to Devin session: https://app.devin.ai/sessions/3d9680ebba5445eba4d8c1301f8d9634
Requested by: @jon-fern

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