"next" should be an optional field in JSON choices

no next => end of the chain

next is mandatory on Choices, we cannot currently end a story with a choice
goal is to fix that

Start by locating where `Choice` objects and story JSON are validated or typed in the TypeScript code, especially any rule that marks `next` as required. Then check where the story-chain parser/loader enforces this requirement and identify the minimal place to allow terminal choices. Run the story parsing/validation tests (or the nearest suite covering choices/chain flow) and confirm success for a choice entry that omits `next` and still ends the chain.

索引モデルが issue の本文から書いたものです。