Large refactor - it compiles!

This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.

pkg/cmd/chat.go

@@ -54,7 +54,7 @@ func ChatCmd(ctx *lmcli.Context) *cobra.Command {
 			if len(args) != 0 {
 				return nil, compMode
 			}
-			return ctx.Store.ConversationShortNameCompletions(toComplete), compMode
+			return ctx.Conversations.ConversationShortNameCompletions(toComplete), compMode
 		},
 	}