[Feature Request] Conversation History Checkpoints

It would be a good idea to have checkpoints in conversation history so that it will be easy to resume/continue from certain points in the interaction, should we inadvertently exit.

From my colleague Shen Li:

> One technique I observed with other tool that I find useful is checkpoint to restore the conversation to some previous point, if have that capability and the context exceed window, you can go back to a previous point and ask it for a summary up to that point, then the context window can just do something like truncate older messages.

This makes a lot of sense, and would be helpful for a customer to be able to successfully resume.

この issue ではファイル、テスト、エントリーポイントが何も指定されていません。まず、会話履歴、再開、コンテキストウィンドウの処理がどこに実装されているかを調査し、その後、作業量を見積もる前にチェックポイントの動作と受け入れ基準を明確にしてください。

索引モデルが issue の本文から書いたものです。