So in order to hire a stable hand at a future point, I need to have the 10,000 AP achievement, apparently.

I currently have 10,036 AP on hand. Does that mean I have the achievement? I can't figure out a way to see if I do.

Also, once I have the achievement, I can use the AP, right? I need to unlock more hairstyles for my HA :P

You just have to have earned it, not have it on hand. You can go spend it all now, and from the point at which you met the achievement onwards, no matter how much AP you have, you can hire a stable hand, but only if you have anough gold.

I'm not sure if the achievement ever gives a notification. One way to check is to see if you have access to the stable hand though?

If you're not sure how to access the stable hand, here's how you can check:
Go to the Stables from your account drop down menu.
When you arrive click on your character portrait.
A pop-up should appear for "Stablehand Manager".

Try seeing if you can contract Farm Maiden Rosemary~
If you can then you're in the clear~ :D

adding to what Imperium said. if for some reason you can't contract her let Krin know since it seems like you've met the requirment and is likely a glitch or bug

I tried hiring the stable hand for three days to see if I could, and that went fine, so I guess I do have the ability! :D

Which is good, because I already spent most of the AP. *chuckles*