docs: add Japanese code of conduct

[kitsuyui]

What / なに

• Add CODE_OF_CONDUCT.ja.md as the Japanese counterpart to CODE_OF_CONDUCT.md.
• Update README.md so the Code of Conduct entry follows the same bilingual pair format as the other community health files.

Why / なぜ

• The repository already keeps CONTRIBUTING, AI_POLICY, and SECURITY as English/Japanese pairs.
• The Code of Conduct now follows the same layout and is easier to find for Japanese readers.

Checklist

• Typo / obvious small fix — no issue needed. Otherwise: linked an agreed issue. / typo・明白な小修正である。それ以外は合意済み issue をリンクした。
• The checks CI runs pass locally (see the repo's README / CI config). / CI と同じチェックがローカルで通る。
• Tests added or updated if behavior changed. / 挙動が変わるならテストを更新した。
• I can explain this change in my own words (AI Policy). / この変更を自分の言葉で説明できる。

Checks:

• npx --yes markdownlint-cli2 README.md CODE_OF_CONDUCT.ja.md
• typos README.md CODE_OF_CONDUCT.ja.md
• git diff --check