I have a Librem 13 v2, running Debian unstable with Mate desktop environment.
Is there a way to automatically adjust the display brightness depending on the environmental light? Does the laptop have an ambient light sensor?
(There was a similar question in 2018 for the Librem 15, but that has no answer).
No, none of the Purism laptops have one.
Could you use the camera as a surrogate ambient light sensor? (provided that the camera is not killed)
An external USB dongle that is an ambient light sensor? Does such a thing exist? I’ve never had a need and hence never gone looking.
All of those ideas sound plausible. You may be able to find some sort of ambient light sensor to interface with various microcontrollers, but I cannot guarantee they are of the USB variety.