we had new pre-primed hardie siding installed about 7 years ago and the painters didn't prime, just did two coats of paint (rodda mid-grade, dont remember the name) no issues that I can see so far except that there is some color fading on one side/wall where it gets a lot of direct sun, it's...
when I had my old shop built a few years back, there was a gap in sheeting at the very top of the ridge, don't remember for sure, may be 2-3 inches and the ridge vent covered it so it would ventilate properly.
nice build and I like how you've incorporated stucco,
any plans for outdoor lights? when I build my last shop, I had to have a porch light by the entrance door...