Consider switching to GraphQL

It makes sense to `gren` to use GitHub GraphQL as it would be much easier to manage and query what we need without different endpoints.

This could also potentially allow #79

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Also coming from #134

Start by tracing gren's current GitHub endpoint usage and reviewing issues #79 and #134 for context. Define the required GraphQL queries and migration scope; done means the project has an agreed implementation plan or a completed switch with release-note behavior preserved.

索引モデルが issue の本文から書いたものです。