feat(skills): add wellarchitected.github.com content to github-app-dev

## Summary

Add references and patterns from https://wellarchitected.github.com to the `github-app-dev` skill.

## Rationale

The GitHub Well-Architected framework provides best practices for GitHub platform integrations that would enhance GitHub App development guidance.

## Scope

- Add wellarchitected.github.com as a reference
- Incorporate framework patterns for app design and security
- Add guidance on webhook handling, authentication, and rate limiting per the framework

## Related Skills

This is part of a broader effort to add wellarchitected.github.com content to:
- cicd-github-actions-dev
- cicd-github-actions-ops
- github-actions-dev
- github-app-dev (this issue)
- iac-terraform-modules-eng
- iac-terraform-orchestration-ops