feat(tools): add list-untranslated script to identify untranslated files

[lacolaco]

Summary

Add a new utility script list-untranslated that identifies files in adev-ja that don't have corresponding .en.* backup files, indicating they haven't been translated yet.

Changes

• ✅ Add tools/list-untranslated.ts - New TypeScript executable script that scans for untranslated files
• ✅ Add pnpm run list-untranslated command to package.json for easy execution
• ✅ Uses simple glob pattern to find all translatable files (.md, .ts, .html, .json)
• ✅ Excludes license files and .en.* backup files from scan
• ✅ Provides clear output showing count and list of untranslated files

Usage

pnpm run list-untranslated

Output example:

Found 149 untranslated files:
  src/content/best-practices/error-handling.md
  src/content/ecosystem/rxjs-interop/output-interop.md
  ...

This tool helps identify translation gaps and prioritize translation work.