Add Japanese translation and automated trilingual sync

[lipingtababa]

Summary

• Full Japanese (ja/) translation of all 44 chapter files, intro, TOC, and contributors
• Automated translation workflow: when a PR changes content in one language, Claude Code translates into the other two languages within the same PR
• Japanese writing quality review job added to the review-writing workflow
• Updated lint.sh and test.sh to validate trilingual chapter sync
• Added CLAUDE.md with translation conventions for the Claude Code action
• Added repo README.md targeting decision makers and software developers
• Fixed GitHub Pages badge URL (was pointing to non-existent GitBook URL)

How the auto-translation works

1. A contributor opens a PR with changes in en/, zh/, or ja/
2. translate.yml detects the source language and triggers Claude Code
3. Claude translates the changed files into the other 2 languages and pushes to the same PR branch
4. The existing review workflow reviews writing quality in all 3 languages
5. Multi-language commits are skipped to avoid translation loops

Test plan

• Verify ja/ renders correctly in HonKit (npm run serve)
• Verify lint passes: npm run lint
• Verify sync check passes: npm test
• Open a test PR with a single-language change and confirm translate workflow triggers