Allow restoring a checkpoint at any point during Agent-based code updates

Currently, it’s only possible to restore a checkpoint when the user inputs a prompt. However, there are situations where we might have partially successful code updates, but then extra unwanted elements get introduced in the later stages. In these cases, it would be very helpful to be able to roll back to a specific line (or checkpoint) while still preserving the context up to that point.

For example, if multiple files are updated, it’s possible to accept only one file and reject the others, but this sometimes still affects subsequent changes due to the context. If we could restore any line directly, we could remove unwanted updates while keeping the progress we want. This would help prevent the rejected changes from influencing future modifications.