feat: Tax settings + unified addon guide + Biteship spec

## 1. Created BITESHIP_ADDON_SPEC.md ✅
- Complete plugin specification
- Database schema, API endpoints
- WooCommerce integration
- React components
- Implementation timeline

## 2. Merged Addon Documentation ✅
Created ADDON_DEVELOPMENT_GUIDE.md (single source of truth):
- Merged ADDON_INJECTION_GUIDE.md + ADDON_HOOK_SYSTEM.md
- Two addon types: Route Injection + Hook System
- Clear examples for each type
- Best practices and troubleshooting
- Deleted old documents

## 3. Tax Settings ✅
Frontend (admin-spa/src/routes/Settings/Tax.tsx):
- Enable/disable tax calculation toggle
- Display standard/reduced/zero tax rates
- Show tax options (prices include tax, based on, display)
- Link to WooCommerce for advanced config
- Clean, simple UI

Backend (includes/Api/TaxController.php):
- GET /settings/tax - Fetch tax settings
- POST /settings/tax/toggle - Enable/disable taxes
- Fetches rates from woocommerce_tax_rates table
- Clears WooCommerce cache on update

## 4. Advanced Local Pickup - TODO
Will be simple: Admin adds multiple pickup locations

## Key Decisions:
✅ Hook system = No hardcoding, zero coupling
✅ Tax settings = Simple toggle + view, advanced in WC
✅ Single addon guide = One source of truth

Next: Advanced Local Pickup locations

includes/Api/Routes.php

@@ -10,6 +10,7 @@ use WooNooW\Api\AuthController;
 use WooNooW\API\PaymentsController;
 use WooNooW\API\StoreController;
 use WooNooW\Api\ShippingController;
+use WooNooW\Api\TaxController;
 
 class Routes {
     public static function init() {
@@ -54,6 +55,10 @@ class Routes {
             // Shipping controller
             $shipping_controller = new ShippingController();
             $shipping_controller->register_routes();
+            
+            // Tax controller
+            $tax_controller = new TaxController();
+            $tax_controller->register_routes();
         });
     }
 }