Update system prompt handling (again)

Add `api.ApplySystemPrompt`, renamed `GetSystemPrompt` to
`DefaultSystemPrompt`.

pkg/cmd/prompt.go

@@ -25,20 +25,10 @@ func PromptCmd(ctx *lmcli.Context) *cobra.Command {
 				return fmt.Errorf("No message was provided.")
 			}
 
-			var messages []api.Message
-
-			system := ctx.Config.GetSystemPrompt()
-			if system != "" {
-				messages = append(messages, api.Message{
-					Role:    api.MessageRoleSystem,
-					Content: system,
-				})
-			}
-
-			messages = append(messages, api.Message{
+			messages := []api.Message{{
 				Role:    api.MessageRoleUser,
 				Content: input,
-			})
+			}}
 
 			_, err = cmdutil.Prompt(ctx, messages, nil)
 			if err != nil {