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).

scripts/build_epub.py

@@ -115,6 +115,8 @@ class EPUBConfig:
     en_subtitle: str = "Master Claude Code in a Weekend"
     zh_title: str = "Claude Code 使用指南"
     zh_subtitle: str = "一个周末掌握 Claude Code"
+    ja_title: str = "Claude Code ハウツーガイド"
+    ja_subtitle: str = "週末で Claude Code をマスターする"
 
     # Cover Settings
     cover_width: int = 600
@@ -1059,10 +1061,10 @@ def main() -> int:
         "--lang",
         type=str,
         default="en",
-        choices=["en", "vi", "zh"],
+        choices=["en", "vi", "zh", "ja"],
         help=(
             "Language code: 'en' for English, 'vi' for Vietnamese, "
-            "'zh' for Chinese (default: en)"
+            "'zh' for Chinese, 'ja' for Japanese (default: en)"
         ),
     )
     parser.add_argument(
@@ -1084,6 +1086,7 @@ def main() -> int:
         "en": (repo_root, "claude-howto-guide.epub", EPUBConfig.en_title),
         "vi": (repo_root / "vi", "claude-howto-guide-vi.epub", EPUBConfig.vi_title),
         "zh": (repo_root / "zh", "claude-howto-guide-zh.epub", EPUBConfig.zh_title),
+        "ja": (repo_root / "ja", "claude-howto-guide-ja.epub", EPUBConfig.ja_title),
     }
     root, default_output_name, title = lang_map[args.lang]
     output = args.output or (repo_root / default_output_name)