How high can the 'skill' of a Hireling go in a Dungeon World game? I see no limit in the rules as written, and at the upper tiers they can have quite a few points (6-10 for City) to distribute between Skill, Loyalty, and secondary Skills. Loyalty is called out as "Starting loyalty higher than 2 is unusual" but nothing for Skill. Is there a limit somewhere else that I'm missing?
Not really, no. There's no fixed limit to each hireling's skill, just the practical limit of having at most 10 points to distribute when the GM creates the hireling.
(Even that practical limits isn't a hard limit. Due to the normal Dungeon World rule that if something happens in the fiction it should be reflected in the numbers, it's even possible to exceed that practical limit during play. If an already-excellent Tracker studies “The Secret Paths of Paws” with the Ascetic Monks of the Deepwood Vale at their remote forest temple at the heart of the World Forest — to use an example made up on the spot — the GM should bump their Tracker skill up by a point.)
You might be thinking skill must have a limit, if you're assuming +skill is similar to something like +INT for PCs. That's not what skill is though. Most hireling moves that reference skill don't give it as a bonus to a roll, so it doesn't have to operate on the same nominal −1 to +3 scale that PCs' stat modifiers are on. The Tracker's skill, for example, is measured against the distance (in rations) for a Perilous Journey; for that kind of application, there's no practical or mechanical breaking point where a Tracker could have “too much” Tracker skill for the mechanics to work. Similarly, the Priest skill is translated into HP healed; the Protector skill translates into an armor bonus; etc. An Adept skill doesn't even get used numerically, instead only being a “how good?” memory-aide for the GM when applying the effects of an Adept's help.
The one exception is the Burglar — that skill does add to rolls, but if you've got a legendary Burglar who walks face-first into a trap, they're going to be “in need of immediate healing”, so they can't get that +7 forward to deal with traps reliably unless they're taking very good care of their Burglar. (And if they don't, they not only don't get that +whatever again soon this dungeon, but they likely won't be getting that bonus later either, since how many new legendary Burglars will want to hire on with a group known for letting their legendary Burglar hirelings die?) The Burglar's bonus to skill checks is quite large, but self-limiting.