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/05-mcp/filesystem-mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "filesystem": {
+      "command": "npx",
+      "args": ["@modelcontextprotocol/server-filesystem", "/home/user/projects"]
+    }
+  }
+}