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

[aRustyDev]

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