Recommend Shopify AI Toolkit in generated theme instructions

[gonzaloriestra]

What

• Recommend the Shopify AI Toolkit in the AGENTS.md generated by shopify theme init.
• Ensure the generated Claude and Copilot adapter files inherit the same recommendation.
• Add a patch changeset for @shopify/theme.

Why

The generated theme instructions require Shopify agent tooling such as learn_shopify_api, but do not tell developers how to install it.

Related to https://github.com/shop/issues-develop/issues/22870

Testing

• pnpm vitest run packages/theme/src/cli/services/init.test.ts
• pnpm eslint packages/theme/src/cli/services/init.ts packages/theme/src/cli/services/init.test.ts
• pnpm prettier --check packages/theme/src/cli/services/init.ts packages/theme/src/cli/services/init.test.ts .changeset/kind-tools-connect.md
• git diff --check

[gonzaloriestra]

This is not the right place, it should be updated in https://github.com/shop/world/pull/976742