To Alex: been there, done that, moved on to Laravel Homestead and Forge on Vagrant, because it's nifty. Thanks anyway.
To Andrew... I actually did follow the mattstauffer.co instructions... and have cycled and recycled through various things I found on stackoverflow et al. Now it's in DO Support's court.
FYI, in case anyone else has problems, the deployment from Forge seems to disable password access, so the keys are necessary, and so server-side fiddling isn't possible. I must admit I'm no expert at this keygen stuff, but followed instructions to the letter, pretty sure.
Thanks... I'll try to get back here with the resolution.