Add GitHub API context adapter

[srkyn]

Summary

• Add a dependency-free GitHub REST API adapter for public repository context
• Generate short review notes for visibility, maintenance state, issues, topics, and license signals
• Document the integration and add test coverage
• Add an MIT license so repository reuse expectations are clear

Verification

• python -m pytest
• python -m compileall src
• python -m src.github_repo_context srkyn/IdentityRiskGraph