Add message branching

Updated the behaviour of commands:

- `lmcli edit`
  - by default create a new branch/message branch with the edited contents
  - add --in-place to avoid creating a branch
  - no longer delete messages after the edited message
  - only do the edit, don't fetch a new response
- `lmcli retry`
  - create a new branch rather than replacing old messages
  - add --offset to change where to retry from

pkg/cmd/continue.go

@@ -26,7 +26,7 @@ func ContinueCmd(ctx *lmcli.Context) *cobra.Command {
 			shortName := args[0]
 			conversation := cmdutil.LookupConversation(ctx, shortName)
 
-			messages, err := ctx.Store.Messages(conversation)
+			messages, err := ctx.Store.PathToLeaf(conversation.SelectedRoot)
 			if err != nil {
 				return fmt.Errorf("could not retrieve conversation messages: %v", err)
 			}