error: Couldn't load public key... : No such file or directory?
fatal: failed to write commit object
First, I removed the format = ssh from my ~/.gitconfig file.
Second, I did some debugging by adding GIT_TRACE=1
at the beginning of the most recent git commit.
In my case, it was:
GIT_TRACE=1 git commit -S -m "3-islower.c: Check for lowercase"
That enabled me to see what GPG is doing, and the log I needed was:
14:34:54.744990 run-command.c:668 trace: run_command: gpg --status-fd=2 -bsau 'my-GPG-key'
My GPG key was incorrect, and I solved that by Telling Git about my GPG Signing Key
My newfound experience enables me to point out that using SSH key to verify commits is a bad idea.