Use linuxCNC as controller for production automation robot

That should be fairly easy with the use of digital inputs and switches to notify Linuxcnc when the part is at whatever position it should be, do whatever has to be done, move to another position etc. Most probably all of it can be controlled in gcode.

I can think of examples that I have seen, but I am coming up blank with Youtube searches.

LinuxCNC can do all that you describe (including the stepping).
In theory LinuxCNC running on the Pi you mention can do everything. (I am not at all clear how you intended to do stepping with a Pi _without_ LinuxCNC) But at this point in time using a Pi is a research project rather than a solution.

You _probably_ want to have a external interface card rather than run a PC and parallel port. (Though that probably would work very well for what you describe, it does smell of old-school and amateur bodging)