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

admin-spa/src/App.tsx

@@ -196,6 +196,7 @@ import SettingsIndex from '@/routes/Settings';
 import SettingsStore from '@/routes/Settings/Store';
 import SettingsPayments from '@/routes/Settings/Payments';
 import SettingsShipping from '@/routes/Settings/Shipping';
+import SettingsTax from '@/routes/Settings/Tax';
 import MorePage from '@/routes/More';
 
 // Addon Route Component - Dynamically loads addon components
@@ -426,7 +427,7 @@ function AppRoutes() {
       <Route path="/settings/store" element={<SettingsStore />} />
       <Route path="/settings/payments" element={<SettingsPayments />} />
       <Route path="/settings/shipping" element={<SettingsShipping />} />
-      <Route path="/settings/taxes" element={<SettingsIndex />} />
+      <Route path="/settings/tax" element={<SettingsTax />} />
       <Route path="/settings/checkout" element={<SettingsIndex />} />
       <Route path="/settings/customers" element={<SettingsIndex />} />
       <Route path="/settings/notifications" element={<SettingsIndex />} />