[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.

The issue names no files, tests, or entry points. First map where conversation history, resumption, and context-window handling are implemented, then clarify checkpoint behavior and acceptance criteria before estimating the work.

Written by the indexing model from the issue text.