I wrote a small Go app that reads from stdin using the technique outlined here: https://flaviocopes.com/go-shell-pipes/
I invoke it like this: cmd | fsjl (where fsjl is my app)
Trouble is, the exit code is always 0 even when cmd exits 1.
How can I "forward" the exit code from cmd?
That's not a job for your Go program. That's a job for whoever's managing the overall pipeline. Pipeline elements don't see each other's exit statuses.
For example, in bash, you can use pipefail:
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.
Here's an example, using a subshell to restrict the effects of pipefail to a single line:
(set -o pipefail && cmd | fsjl)
You can't always assume bash, but there are ways to do similar things in other shells.