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/billing-automation/SKILL.md

@@ -535,32 +535,3 @@ class UsageBillingEngine:
 
         return charge
 ```
-
-## Resources
-
-- **references/billing-cycles.md**: Billing cycle management
-- **references/dunning-management.md**: Failed payment recovery
-- **references/proration.md**: Prorated charge calculations
-- **references/tax-calculation.md**: Tax/VAT/GST handling
-- **references/invoice-lifecycle.md**: Invoice state management
-- **assets/billing-state-machine.yaml**: Billing workflow
-- **assets/invoice-template.html**: Invoice templates
-- **assets/dunning-policy.yaml**: Dunning configuration
-
-## Best Practices
-
-1. **Automate Everything**: Minimize manual intervention
-2. **Clear Communication**: Notify customers of billing events
-3. **Flexible Retry Logic**: Balance recovery with customer experience
-4. **Accurate Proration**: Fair calculation for plan changes
-5. **Tax Compliance**: Calculate correct tax for jurisdiction
-6. **Audit Trail**: Log all billing events
-7. **Graceful Degradation**: Handle edge cases without breaking
-
-## Common Pitfalls
-
-- **Incorrect Proration**: Not accounting for partial periods
-- **Missing Tax**: Forgetting to add tax to invoices
-- **Aggressive Dunning**: Canceling too quickly
-- **No Notifications**: Not informing customers of failures
-- **Hardcoded Cycles**: Not supporting custom billing dates