Add typed JSON output to theme check

[gonzaloriestra]

WHY are these changes introduced?

Theme Check JSON needs consistent validation results and diagnostic fields.

WHAT is this pull request doing?

Return valid, flat issues and consistent severity counts with absolute file paths and zero-based camelCase positions. Retain completed validation in environment batches, honor the blocking policy, and wait for requested corrections before final JSON.

The public output follows docs/cli/json-output.md; explicitly named environments use one {environments: [...]} object.

How to manually test your changes?

shopify theme check --path . --json
shopify theme check --path . --output=json
shopify theme check --json-schema

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[github-actions]

⚠️ Potential Breaking Changes Detected

This PR contains changes that may break the existing contract.

@shopify/dev_experience — this PR contains breaking changes that require coordination for the next major release.

📦 Major Version Changesets

The following changesets request a major version bump:

Changeset	Package
theme-check-json-contract.md	'@shopify/cli': major
theme-duplicate-json-contract.md	'@shopify/cli': major
theme-info-json-contract.md	'@shopify/cli': major
theme-list-json-contract.md	'@shopify/cli': major
theme-open-json-contract.md	'@shopify/cli': major
theme-preview-json-contract.md	'@shopify/cli': major
theme-push-json-contract.md	"@shopify/cli": major

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/public/node/base-command.d.ts

@@ -35,6 +35,7 @@ declare abstract class BaseCommand extends Command {
         argv: string[];
     }>;
     protected environmentsFilename(): string | undefined;
+    protected validateNonTTYFlags(flags: FlagOutput): void;
     protected failMissingNonTTYFlags(flags: FlagOutput, requiredFlags: string[]): void;
     private failMissingNonTTYFlagRequirements;
     private applicableNonTTYFlagRequirements;

packages/cli-kit/dist/public/node/themes/theme-manager.d.ts

@@ -11,5 +11,12 @@ export declare abstract class ThemeManager {
     findOrCreate(name?: string, role?: Role): Promise<Theme>;
     fetch(name?: string, role?: Role): Promise<Theme | undefined>;
     generateThemeName(context: string): string;
-    create(themeRole?: Role, themeName?: string): Promise<Theme>;
+    create(themeRole?: Role, themeName?: string): Promise<{
+        createdAtRuntime: boolean;
+        id: number;
+        name: string;
+        processing: boolean;
+        role: string;
+        src?: string;
+    }>;
 }
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