Success! I knew that editing /etc/network/interfaces was important but I just couldn’t find an explanation anywhere that clarified the file’s syntax or what each of the short “words” in it meant. Then I found this page on the Debian wiki (specifically step 4, ‘Using DHCP to automatically configure the interface’) and that helped a great deal.
I knew from ls /sys/class/net that my available network interfaces were
eno1 lo wlp1s0
…where eno1 is the ethernet. (I did all of this with an ethernet cable plugged in.)
So I opened /etc/network/interfaces in nano, and edited it both to look like step 4 on the Debian wiki but replaced their example eth0 with my eno1:
$ nano /etc/network/interfaces # Some text removed to show the important bits
auto eno1
allow-hotplug eno1
iface eno1 inet dhcp
iface eno1 inet6 auto
If I remember correctly, inet is IPv4, so I put dhcp on the end of that line to allow it to allocate a private IP address for my server by itself.
I included inet6 (IPv6) and auto for good measure, I doubt I’m going to use anything on IPv6.
Now, when I type hostname -I I get an IP address!
$ hostname -I
192.168.0.112
I cannot view the admin login page via yunohost.local on a separate device, but I can view it when I type 192.168.0.112 into the browser instead, so I consider that success. 
I have rebooted several times, and it all persists across sessions, so that’s good.
I did not need to install lshw rfkill iw or any of those other programs after all, but trying to do so indicated to me that the internet connection was working.
I’m going to keep it connected to the router via ethernet cable, as it seems that’s what Yunohost “prefers” instead of wi-fi.
Thank you everyone for your help.