feat(i18n): Add Japanese (ja/) translation (#105)

- Translate all 101 markdown files: P1 core, all 10 modules, examples,
  auxiliary docs (CONTRIBUTING, CODE_OF_CONDUCT, SECURITY, CLAUDE.md, etc.),
  peripheral docs (.github/, docs/, resources/, scripts/)
- Translate comments and user-facing messages in 06-hooks/*.sh examples
- Copy 05-mcp/*.json examples (standard JSON, no comments)
- Update root README.md language switcher to include 日本語
- Add ja/TRANSLATION_NOTES.md (glossary + style guide)

All translations pass pre-commit quality gates (markdown-lint,
cross-references, mermaid-syntax, link-check, build-epub).

ja/RELEASE_NOTES.md

@@ -0,0 +1,15 @@
+<!-- i18n-source: RELEASE_NOTES.md -->
+<!-- i18n-source-sha: 7f2e773 -->
+<!-- i18n-date: 2026-04-27 -->
+
+## v2.2.0 — 2026-03-26
+
+### バグ修正
+
+- CI のリンクチェックで失敗していた marketplace の死リンクを削除（3fdf0d6）
+- `sandboxed` と `pycache` を cSpell 辞書に追加（dc64618）
+
+**Full Changelog**: https://github.com/luongnv89/claude-howto/compare/v2.1.0...v2.1.1
+
+---
+**Last Updated**: April 9, 2026