Respect user .theme-check.yml in theme app extension builds

[erikmay]

Summary

runThemeCheck in packages/app/src/cli/services/build/theme-check.ts hardcodes the config identifier 'theme-check:theme-app-extension', which bypasses theme-check-node's user-config discovery (findConfigPath). As a result, a .theme-check.yml placed in the theme app extension root is ignored during shopify app dev and shopify app deploy, even though the standalone shopify theme check command respects it.

This PR makes runThemeCheck check for .theme-check.yml in the extension directory and pass undefined to check() when present, letting theme-check-node auto-discover the user config. When absent, the existing bundled theme-check:theme-app-extension defaults are applied exactly as before — no behavior change for extensions without a user config.

Motivation

Theme app extensions only allow a fixed set of top-level directories (assets/, blocks/, snippets/, locales/, config/). Authors who want a separate source directory (e.g. uncompiled Liquid / TS / Tailwind that gets built into blocks/ + assets/) currently have two options:

1. Dot-prefix the source dir (.src/) so theme-check's **/*.{liquid,json} glob skips it.
2. Fork the CLI.

Option 1 works but is surprising and fragile. Option 2 is heavy. With this change, a plain src/ source directory works as expected:

# extensions/my-extension/.theme-check.yml
extends: theme-check:theme-app-extension
ignore:
  - "src/**"
  - "node_modules/**"

Changes

• packages/app/src/cli/services/build/theme-check.ts: check for .theme-check.yml and fall through to user-config discovery when present.
• Added a changeset (@shopify/app patch).

Test plan

• Theme app extension without .theme-check.yml — shopify app dev and shopify app deploy produce identical theme-check output to before (uses bundled theme-app-extension defaults).
• Theme app extension with .theme-check.yml extending theme-check:theme-app-extension and adding ignore: ['src/**'] — shopify app dev skips src/** offenses.
• Same .theme-check.yml with a custom check severity override — shopify app dev reflects the override.