Global assistant: restore the workflow to any point a conversation reached

Undo only reaches the newest reply. A change from a few turns back cannot be taken out without re-prompting, and the assistant has usually made several changes by then.

The data is already there. Every global reply carries the workflow as it stood before it, and its own YAML for putting it back, so reaching an older point is a matter of tracking a position rather than a single toggled reply.

Two things stopped us doing it in https://github.com/OpenFn/lightning/pull/5119. Restoring rewrites the shared document under every collaborator and nothing warns them; the current undo already does this, it just is not obvious because it only ever moves one step. And restoring a workflow to a past state is what the versions work is for, so a chat-scoped history built separately would be a second mechanism disagreeing with it.

Worth picking up alongside versions rather than before it. The naming matters too: "undo" is honest for the newest reply and misleading for an older one, where going back also drops everything after it.

Start by reading pull request #5119 and the repository's versions work, since this issue explicitly depends on that design. Trace how global replies retain prior workflow state and YAML, then define completion around restoring any reached point, warning collaborators about shared-document changes, and distinguishing older-point restoration from the current newest-reply undo.