I was hoping thats what you would say about the braces ! best on all four faces ideally.
The way I would do it? Well first of all don't take any of this as any sort of criticism because its not, I am a woodworker and there is certainly nothing wrong with what you have done but the braces are simple in concept but can be made and fixed in many ways
The sides parallel with the shed are the ones that would "rack" first, either towards or away from the shed...This is because the only thing stopping this is the two coachscrew fixings at the top of the posts where the rail is fixed..... You have already said you know it needs braces, so how to do it for best rigidity.........
The rails are fixed to the side of the posts so the brace will have to be lined up central with either the post or the rail. The strongest way with the least carpentry ( not so easy once its built !) is to fix the brace to the side of the post which will bring the brace 3x2 in line with the top rail 3x2....With me? so the brace and top rail sides are flush both sides?
now the top rail and brace can be fixed simply with nails or screws alone but I would cut out a small birdsmouth notch so the brace is let into the beam about 1". This means the angles are not quite 45% but you could leave the post end overlong and saw it flush when you are happy with the lineup and toprail notch fit.
The other way, along the length is still prone to racking though this is lessened by the top 3 straddle rails, so I would put the braces to all four sides.
There is an order to this, it will be much easier if you fix the braces to the top straddle rails first, that is, to the sides parallel with the shed..........
That way when you have cut the rails to length flush with the post you will have nothing in the way for the braces on the other sides....
I hope thats clear to you, easy to show but harder to explain !
What grapevine are you planting? can you recommend one for growing outside? I have seen them but only old ones that people can't remember the name of.