docs: add testing.md references to development-guideline and architecture

Add cross-references to testing.md in:
- development-guideline.md: Testing Guidelines section
- architecture.md: テスト戦略と依存関係 section

Makes testing strategy more discoverable when reading development docs.

Author: claude

docs/architecture.md

@@ -823,6 +823,8 @@ background.ts (実行)
 
 ## テスト戦略と依存関係
 
+> **📖 包括的なテスト戦略については [testing.md](testing.md) を参照してください。このセクションはテストのアーキテクチャ的側面に焦点を当てています。**
+
 ### ユニットテストの依存関係
 
 ```mermaid

docs/development-guideline.md

@@ -481,6 +481,10 @@ const sanitizedTitle = title.replaceAll(/[\[\]]/g, '');
 
 ## Testing Guidelines
 
+> **📖 For comprehensive testing strategy including coverage goals, test layers, tools, and implementation roadmap, see [testing.md](testing.md)**
+
+This section covers practical testing guidelines for writing tests.
+
 ### Test Structure
 
 ```javascript