docs: add native Japanese translation of ECC documentation (ja-JP)

Translate everything-claude-code repository to Japanese including:
- 17 root documentation files
- 60 agent documentation files
- 80 command documentation files
- 99 rule files across 18 language directories (common, angular, arkts, cpp, csharp, dart, fsharp, golang, java, kotlin, perl, php, python, ruby, rust, swift, typescript, web)
- 199 skill documentation files

Total: 455 files translated to Japanese with:
- Consistent terminology glossary applied throughout
- YAML field names preserved in English (name, description, etc.)
- Code blocks and examples untouched (comments translated)
- Markdown structure and relative links preserved
- Professional translation maintaining technical accuracy

This translation expands ECC accessibility to Japanese-speaking developers and teams.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/ja-JP/rules/golang/testing.md

@@ -0,0 +1,31 @@
+---
+paths:
+  - "**/*.go"
+  - "**/go.mod"
+  - "**/go.sum"
+---
+# Go テスト
+
+> このファイルは [common/testing.md](../common/testing.md) を Go 固有のコンテンツで拡張します。
+
+## フレームワーク
+
+標準の `go test` と**テーブル駆動テスト**を使用する。
+
+## 競合検出
+
+常に `-race` フラグを付けて実行する:
+
+```bash
+go test -race ./...
+```
+
+## カバレッジ
+
+```bash
+go test -cover ./...
+```
+
+## リファレンス
+
+スキル: `golang-testing` で詳細な Go テストパターンとヘルパーを参照してください。