feat(library): add Stripe billing and failed-payment agent guide

## Summary
- Add a Stripe billing guide and cover image for failed-payment classification, alerts, and approval.
- Document the Stripe Webhook trigger, event selection, signing secret, deployment, and Stripe retrieval operations with invoice-ID mapping.
- Keep payment and retry policy in Stripe and describe Sim's capabilities without superlatives.

## Type of Change
- [x] Feature

## Testing
- bun run lint
- bun run type-check
- bun run check:audits
- bun run test
- Block-registry audit and focused library-content audit

## Checklist
- [x] Code follows project style guidelines
- [x] Self-reviewed my changes
- [ ] Tests added/updated and passing
- [x] No new warnings introduced
- [x] I confirm that I have read and agree to the terms outlined in the [Contributor License Agreement (CLA)](./CONTRIBUTING.md#contributor-license-agreement-cla)

Author: icecrasher321