Teach Hydrogen templates and skills about mock.shop's store catalog

[BobbyNguye]

Preview-branch version of #3984 and #3985. Those target main, which is frozen; this re-does the work for Hydrogen 3, where there is no scaffolding CLI, so the change lives in the templates, the packaged skills, the checkout interceptor, and the example app.

TL;DR: mock.shop is not one store. It is a catalog of fictional stores, each on its own host, listed at https://mock.shop/llms.txt. Hydrogen treated it as a single endpoint, so anyone building without a store always got the default apparel catalog and no agent knew there was a choice. This PR lets developers pick a store with the store-domain env var they already have, and teaches agents that the catalog exists.

Before

// templates/react-router/app/lib/storefront.ts
const storeDomain = usingMockShop ? "mock.shop" : getStoreDomain(env);

In mock mode PUBLIC_STORE_DOMAIN was ignored. The skills described mock.shop only as "all tokenless access supports".

After

const storeDomain = usingMockShop ? getMockShopDomain(env) : getStoreDomain(env);
// PUBLIC_STORE_DOMAIN=pets.mock.shop -> that store's catalog, still mock mode
// PUBLIC_STORE_DOMAIN=            -> the default store at mock.shop

What this changes

• Core: cart permalinks hand off to demostore.mock.shop for every *.mock.shop host, not only the bare mock.shop. Per-store hosts do not render /cart/..., so the old check sent buyers to a 404.
• React Router template: a mock.shop host in PUBLIC_STORE_DOMAIN selects that store and implies mock mode. .env.example, the README, and AGENTS.md ("No store yet?") explain the directory.
• Next.js template: the same through NEXT_PUBLIC_STORE_DOMAIN. The mock-mode warning names the store in use and points at the directory.
• Skills: hydrogen-storefront-client and hydrogen-setup explain that mock.shop is a catalog and how to pick a store whose categories match what the user is building.
• Hydrogen example: the homepage notice says how to pick another store, the isShopLinked gate keeps showing it for per-store hosts (still mock data), and a mockShopStore e2e env plus smoke spec boot the example against pets.mock.shop.

Developer impact

Includes a patch changeset for @shopify/hydrogen: the checkout interceptor fix and the skill guidance. No new exports and no runtime contract changes. Templates are in the changeset ignore list, so their changes carry no bump.

One behaviour change in the React Router template: PUBLIC_STORE_DOMAIN=<store>.mock.shop together with a private token now runs in mock mode (mock buyer IP, no storefront ID, well-known mock token) instead of real-store mode. .myshopify.com domains are unaffected.

UX impact

The example app's homepage notice has new copy and links to https://mock.shop/llms.txt. The templates have no visible change unless a store is chosen.

Out of scope

• No interactive store picker. preview has no init, so choosing a store is an env-var edit guided by docs and skills. Let init pick a mock.shop store #3985 has no equivalent here.
• No new env var. Reusing the store-domain variable matches the example app and the shopify.dev mock.shop page.

Risk

• The mock.shop host check is a suffix match on .mock.shop, duplicated in core and both templates instead of exported from core, to keep templates dependency-light.
• The mockShopStore smoke spec depends on pets.mock.shop staying published with a collection image and product images. It has both today.

How to Test

1. Run pnpm install && pnpm build:pkgs.
2. In templates/react-router, run cp .env.example .env, set PUBLIC_STORE_DOMAIN=pets.mock.shop, and run pnpm dev.
3. Open the printed local URL. The catalog is the "Paws and Whimsy" pet store, not apparel.
4. Clear PUBLIC_STORE_DOMAIN and reload. The default apparel catalog is back.
5. In templates/nextjs, run cp .env.example .env, set NEXT_PUBLIC_STORE_DOMAIN=pets.mock.shop, and run pnpm dev. Open http://localhost:3000. The server log says Running against mock.shop (pets.mock.shop) and the catalog is the pet store.
6. In examples/hydrogen, run pnpm e2e:smoke -g "mock.shop store". The spec boots the example against pets.mock.shop, sees the notice with the directory link, and finds products.

[fredericoo]

very good! works fine for several mock shops