Add typed JSON output to Hydrogen build tooling

[gonzaloriestra]

WHY are these changes introduced?

Build, route validation, and code generation need typed results that scripts can consume.

Fixes https://github.com/shop/issues-develop/issues/23677

WHAT is this pull request doing?

Add strict schemas and --json to build, check, and codegen. Return absolute artifact paths, a validation valid field, and named generated files with source patterns. Keep diagnostics on stderr and reject finite JSON output with watch mode.

HOW to test your changes?

shopify hydrogen check routes --json
shopify hydrogen codegen --json
shopify hydrogen build --json

Checklist

• I have read the contributing guidelines
• I have considered cross-platform impacts
• I have added a changeset for user-facing changes
• I have added tests
• I have updated the documentation

[shopify]

Oxygen deployed a preview of your gonzalo/hydrogen-json-build branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	October 14, 2026 9:54 AM

Learn more about Hydrogen's GitHub integration.

[fredericoo]

nice and tidy - pulling checkRoutes out as a pure function returning the domain result is a good shape, and the human output looks unchanged. A few small comments, mainly Vite's TTY progress output possibly getting into stdout in JSON mode, and codegen detecting --json differently from build/check.