feat(mail): accept multiple message IDs in delete command

Allow `gt mail delete` to accept multiple message IDs at once,
matching the existing behavior of archive, mark-read, and mark-unread.

Also adds --body as an alias for --message in mail reply.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/mail_thread.go

@@ -80,6 +80,9 @@ func runMailThread(cmd *cobra.Command, args []string) error {
 }
 
 func runMailReply(cmd *cobra.Command, args []string) error {
+	if mailReplyMessage == "" {
+		return fmt.Errorf("required flag \"message\" or \"body\" not set")
+	}
 	msgID := args[0]
 
 	// Get message body from positional arg or flag (positional takes precedence)