Do Cittamatra / Yogacara explicitly refute the existence of an external world?

No. Nothing exists as we perceive it. We are seeing through eyes, hearing through ears, never experiential: or first hand. So essentially the world does not exist as we think it does, so is that non-existent or different?

I think Cittamatra texts do explicitly refute it. Vasubhandu's Twenty Stanzas and autocommentary is a text that gives logical arguments against the presence of an external world. The very begining of this text reads:

"The three realms are consciousness only" of the Mahayana is established through the scriptural expression, "the three realms are mind only." "Mind," "thought," "consciousness," and "perception" (uijñapti) are different names. Here, "mind" and "thought" are lumped together with mental activities (caitta). "Only" excludes objects of perception (artha) that are external to consciousness, not associates of consciousness. When internal consciousness is born, it appears resembling external objects of perception, I but in the same way that one with diseased eyesight sees nonexistent hairs, flies, etc. Here there is not the slightest aspect of reality.

and the text goes on to refute various objections to the nonexistence of external objects.

Also, in his autocommentary on the Thirty Verses he writes:

If there is nothing at all but consciousness and no external condi- tions at all, from what are the many imaginations born? The Verses say,

From the consciousness that is all seeds Transformation [occurs] in such - and -such ways. Due to the power of mutual influence, That- and -that imagination is born. (18)

Even though internal consciousness exists, still, there are no external conditions, so what is the basis for the continuity of birth and death among sentient beings? The Verses say,

The habit energy of various actions Together with the habit energy of the two graspings, When prior retribution is exhausted, Subsequently produce other retribution. (19)

Asanga's Mahayana-Samgraha is probably another good place to look but I haven't read that text yet so I'm not sure if it is explicit.

Downplaying the external world (is "external world" quite the same as 'form (rūpa)'?), saying that our ideas of it are reified fabrications of the mind created from (or dependent on) sense-contact, seems to me a feature even of the Pali suttas ... with the proviso that nihilism (e.g. "There is nothing given, nothing offered, etc."), and presumably solipsism, are identified as a "wrong view".

Distinguishing or reconciling these views is I think known as the 'two truths' doctrine[s].

The following answer is not expert; it's slightly referenced but just recently researched because of your question.

The Theory of Two Truths in India seems to me to support the thesis (that Yogacara explicitly refute the existence of an external world).

Some quotes from this article are:

The central thesis in the Yogācāra philosophy, the theory of the two truths echoes this, is the assertion that all that is conventionally real is only ideas, representations, images, creations of the mind, and that there is no conventionally real object that exists outside the mind to which it corresponds. These ideas are the only objects of any cognition. The whole universe is a mental universe.

Apart from those representations, consciousnesses, which appear to be external objects, there is no conventionally real external content which corresponds to what appears.

All these arguments based on the facts of experience show that the objects do not exist really outside the mind, that they are the products of mental creation and that their appearance is entirely mind dependent. Therefore Yogācāra's theory of the two truths concludes that the whole world is a product of mind—it is the collective mental action (karma) of all beings. All living beings see the same world because of the identical maturation of their karmic consequences. Since the karmic histories of beings are same, there is h*m*geneity in the way in which the world is experienced and perceived. This is the reason there is an orderly world instead of chaotic and arbitrariness. This is also the reason behind the impressions of the objectivity of the world.

I expect you'll want to investigate the author's claims in more detail (using the references which he cites).

Conversely Yogācāra Critiques of the Two Truths seems to me to present a different picture, in which the Yogacara aren't so external-world-doesn't-exist extreme:

These remarks of Vasubandhu’s are probably the very first attempt on the Yogācāra side to incorporate the two truths into their more complicated structure of the three natures. Based on these remarks, we can draw the following diagram:

                      emptiness of persons & dharmas —existent(?)
ultimate truth —
                      no persons or dharmas —nonexistent(?)


                      dependent nature —existent(?)
conventional truth —
                      imagined nature —nonexistent(?)

As we see, the conventional truth is described as having two aspects. Viewed as the imagined nature (parikalpita-svabhāva), it does not exist; while viewed as the dependent nature (paratantra-svabhāva), it does exist. So conventional reality cannot be one-sidedly taken as purely imaginary or illusory; this would be to fall into nihilism. Nor can conventional reality be taken as utterly existent, on the other hand, because the imagined nature does not exist. That is why the Yogācāra criticizes both of the extremes into which his Madhyamaka opponent tends to fall.

According to this paper, the Yogacara doctrine isn't just that the conventional (external?) world is imaginary: it has a merely-imaginary (unreal) component but also a dependent (existent albeit empty) component.

And allegedly the Yogācāra criticizes and accuses their "Madhyamaka opponents" of nihilism and extremism, which implies that they hold their own doctrine to be innocent of such extremism.

In conclusion:

One of the goals of the Yogācāra theory of the three natures was to improve on this two-tiered paradigm, and to restore a more robust and holistic worldview. My study of some scattered sources from Maitreyanātha, Asaṅga, and Vasubandhu has demonstrated that they criticized the Madhyamaka version of two truths doctrine on the basis of the Yogā- cāra theory of the three natures.

So the Yogācāra develop the doctrine of The Three Natures, one of which is the world as it's understood according to the theory of dependent origination.

According to Kagyu understanding, Yogacara's position isn't metaphysical, it is methodological. It's not that the external world does not exist, it is that our experience of the world is 100% always represented by organs and/or mind, there is no experience outside of that, so practically speaking mind-made experience is "all we have"*

Which is why trying to figure out how exactly the experience is generated and mastering the practical skills for working with mind and experience is of key importance in Yogacara (comparing to the earlier Buddhism's focus on discipline/ethics and the Madhyamika's focus on philosophy).

Yogacarins put so much emphasis on the mind because they are concerned with practice. Some people have thought they were saying that everything is in your mind. Some of these people were influenced by certain Western philosophies. -- _{Traleg Kyabgon Rinpoche}

Generally speaking Kaguypas seem to prefer to stay away from philosophical discussions and focus on practice, which makes sense given their self-identification as "the practice lineage":

We are not concerned about whether or not a chair exists. That's secondary. Why spend so much time arguing about whether or not a chair exists, instead of learning how we experience and relate to the chair, and what sort of experience we've got? -- _{Traleg Kyabgon Rinpoche}

*_{Using the modern language we could say, reality is an interpretation we make, there is no reality to be experienced outside of an interpretation. In other words, there is no absolute truth separate from the conventional truth.}