Add --country flag to store create dev

[amcaplan]

What

Adds a --country flag to shopify store create dev, implementing the country piece of shop/world#22968.

This extracts and shares the country-code validation that already exists for store create preview:

• flags.ts now exports the shared countryFlag(env) factory (normalizes input to trimmed uppercase), the isCountryCode() validator, and a new shared invalidCountryCodeMessage constant so both commands report identical guidance.
• devStoreFlags.country added (env SHOPIFY_FLAG_STORE_COUNTRY) alongside the existing previewStoreFlags.country.
• store create preview now uses the shared invalidCountryCodeMessage (removed the duplicated string).
• store create dev validates --country and threads it into createDevStore, surfacing it in JSON and success output.

Draft — why

This is intentionally flag scaffolding only. country is not yet sent to the BP createAppDevelopmentStore mutation because the published schema does not expose the argument until the backend lands:

• Backend PR: shop/world#671185 (part of shop/world#22968) — not yet merged to main.
• The CLI's graphql-codegen fetches the schema from shop/world main, and CI enforces a freshness check. Wiring country onto the operation now would fail that check.

The flag is validated end-to-end locally (tophatted against a local BP that accepts country), so the wiring is a trivial follow-up.

Follow-up (once shop/world#671185 merges)

1. pnpm graphql-codegen:get-graphql-schemas && pnpm graphql-codegen
2. Add $country: String to create_app_development_store.graphql and the country: $country argument.
3. Replace the NOTE in services/store/create/dev.ts with country: options.country.
4. Add a changeset and unhide the command when the broader feature is ready.

Notes

• Command is hidden; no changeset (not functional end-to-end yet).
• Tests: 36 pass (command + service). Type-check and lint clean.