feat: enforce SKU validity, use SKU as handle

This commit enforces proper SKU validation, uses the SKU as the Shopify handle, and implements ID-based product updates to allow renaming. It also extracts the IShop interface for TDD.

docs/ARCHITECTURE.md

@@ -37,6 +37,18 @@ To avoid hitting Shopify API rate limits and Google Apps Script execution time l
     - Reads `pendingEdits`.
     - Filters for edits older than `BATCH_INTERVAL_MS` (30s) to allow for multiple quick edits to the same SKU.
     - Iterates through valid edits and calls `Product.UpdateShopifyProduct`.
+    - **SKU Validation**: Before any action, checks if the SKU is valid (not empty, `?`, or `n`). Aborts if invalid.
+
+### 2. Product Lifecycle Logic
+
+- **Creation**:
+    - Uses the **SKU** as the Shopify **Handle** (URL slug).
+    - Prevents creation if the SKU is a placeholder.
+
+- **Updates**:
+    - Prioritizes **ID-based lookup**.
+    - If the `shopify_id` column is populated, the system trusts this ID to locate the product in Shopify, even if the SKU has changed in the sheet.
+    - As a result, changing a SKU in the sheet and syncing will **rename** the existing product (handle/SKU) rather than creating a duplicate.
 
 ### 2. Shopify Integration (`src/shopifyApi.ts`)
 
@@ -45,7 +57,9 @@ The project uses a hybrid approach for the Shopify Admin API:
 - **REST API**: Used primarily for fetching Orders (legacy support).
 - **GraphQL API**: Used for fetching and updating Products and Inventory.
 
-The `Shop` class handles authentication using credentials stored in the "vars" sheet.
+- **GraphQL API**: Used for fetching and updating Products and Inventory.
+
+The `Shop` class implements the `IShop` interface, handling authentication using credentials stored in the "vars" sheet. The interface decoupling facilitates robust unit testing via `MockShop`.
 
 ### 3. Configuration (`src/config.ts`)