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/list.go

@@ -20,7 +20,7 @@ func ListCmd(ctx *lmcli.Context) *cobra.Command {
 		Short:   "List conversations",
 		Long:    `List conversations in order of recent activity`,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			messages, err := ctx.Store.LatestConversationMessages()
+			messages, err := ctx.Conversations.LatestConversationMessages()
 			if err != nil {
 				return fmt.Errorf("Could not fetch conversations: %v", err)
 			}