Add shared GitHub repository config checks

[ineedjet]

🤖 [rubykatzen/baseline]:

Summary

• add config/github.yml with expected GitHub repository settings
• add a generic checker and version-matched composite action
• export github-shared.yml and use it from baseline through github.yml
• document consumer setup and JSON skip exceptions

Initial checks

• wiki disabled
• auto-merge enabled
• merged branches deleted automatically

Verification

• 20 unit tests
• full pre-commit suite
• live check against rubykatzen/baseline

Closes #103