gpg: decryption failed: No secret key
OMG! Where's my secret key gone!?
But but but, gpg –list-secret-keys says they're there. Puzzled…
Ray Oei's Stack Overflow answer solved the mystery for me: pinentry never got invoked, so likely something's up with the agent… Killing (and thus restaring) the gpg-agent did the trick:
gpgconf --kill gpg-agent
Thank you internet stranger. Balance restored.