docs: add English and Japanese contributor guides

[cotore-game]

Summary

• make README.md the English project guide
• add README-JA.md with setup and usage instructions in Japanese
• add CONTRIBUTING.md with development, testing, pull request, and language contribution guidance
• include the additional documents in installations and release archives
• run the test workflow for pull requests targeting develop as well as main

Verification

• 98 specification tests passed (1 ZIP test skipped locally because zip is not installed)
• locale catalog validation passed
• Docker-based ShellCheck passed

This documentation and CI-only change does not require a version bump.