docs: document testing and coverage requirements

docs/ARCHITECTURE.md

@@ -87,13 +87,20 @@ A dedicated side panel provides visibility into the background queue system.
     - Provides controls to globally enable/disable processing.
     - Allows manual intervention (delete/push) for individual items.
 
-### 6. Service Layer & Testing
+### 6. Service Layer, Testing & Quality
 
 To enable unit testing without Google Apps Script dependencies, the project uses a Service pattern with Dependency Injection.
 
+#### Architecture
 - **`ISpreadsheetService`**: Interface for all sheet interactions.
 - **`GASSpreadsheetService`**: Production implementation wrapping `SpreadsheetApp`.
 - **`MockSpreadsheetService`**: In-memory implementation for tests.
 
+#### Quality Assurance
+We use **Husky** and **lint-staged** to enforce quality standards at the commit level:
+1.  **Pre-commit Hook**: Automatically runs `npm test -- --onlyChanged --coverage`.
+2.  **Coverage Policy**: Any file modified in a commit must meet an **80% line coverage** threshold. This ensures the codebase quality improves monotonically ("Boy Scout Rule").
+
+
 Classes (like `Product`) should accept an `ISpreadsheetService` in their constructor. This allows providing the Mock service during tests to verify logic without touching real Google Sheets.