fix(skills): remove phantom resource references and fix CoC links (#447)

Remove references to non-existent resource files (references/, assets/,
scripts/, examples/) from 115 skill SKILL.md files. These sections
pointed to directories and files that were never created, causing
confusion when users install skills.

Also fix broken Code of Conduct links in issue templates to use
absolute GitHub URLs instead of relative paths that 404.

plugins/payment-processing/skills/paypal-integration/SKILL.md

@@ -448,31 +448,3 @@ def test_payment_flow():
     # captured = client.capture_order(order['id'])
     # assert captured['status'] == 'COMPLETED'
 ```
-
-## Resources
-
-- **references/express-checkout.md**: Express Checkout implementation guide
-- **references/ipn-handling.md**: IPN verification and processing
-- **references/refund-workflows.md**: Refund handling patterns
-- **references/billing-agreements.md**: Recurring billing setup
-- **assets/paypal-client.py**: Production PayPal client
-- **assets/ipn-processor.py**: IPN webhook processor
-- **assets/recurring-billing.py**: Subscription management
-
-## Best Practices
-
-1. **Always Verify IPN**: Never trust IPN without verification
-2. **Idempotent Processing**: Handle duplicate IPN notifications
-3. **Error Handling**: Implement robust error handling
-4. **Logging**: Log all transactions and errors
-5. **Test Thoroughly**: Use sandbox extensively
-6. **Webhook Backup**: Don't rely solely on client-side callbacks
-7. **Currency Handling**: Always specify currency explicitly
-
-## Common Pitfalls
-
-- **Not Verifying IPN**: Accepting IPN without verification
-- **Duplicate Processing**: Not checking for duplicate transactions
-- **Wrong Environment**: Mixing sandbox and production URLs/credentials
-- **Missing Webhooks**: Not handling all payment states
-- **Hardcoded Values**: Not making configurable for different environments