What changed

• add a Japanese translation at README.ja.md
• add language links in README.md and README.zh-CN.md so the new translation is discoverable

Why

• the repository already includes English and Simplified Chinese documentation, and this adds a Japanese README with the same core onboarding and protocol information

User impact

• Japanese readers can understand the project overview, protocol, quick start, daemon behavior, and command reference without relying on machine translation

Validation

• reviewed the translated markdown content
• ran git diff --check