Why is there a negation of "ability" but not a negation of "agility"?
The OED sets out several operational senses of dis-:
- Undoing (disown)
- Depriving (disarm)
- Reversing (dishonest)
- Imputing a negative feeling (disapprove)
The prefix un- (from Old English) almost always signifies a simple negation and often distinguishes itself from its Latin cousins in-, im-. So to be unmoral (i.e, to be amoral) is have no ethical guidelines, while to be immoral is be actively evil.
Likewise, there are differences in meaning from the prefix dis-. If you're unarmed, then you have no weapons, but if you've been disarmed, you once had them and someone has taken them away. (See 2 above.) The appropriations committee may unapprove a project, removing its funds and undoing a previous action for fiscal reasons, but if the members disapprove of the project, they actively dislike it. (See 4 above.)
Unfortunately, if the words are in use, there are no rules that will tell you the connotations. For instance, disapprove once had the additional meaning of disprove, but that meaning is obsolete, superseded by the sense of frowning upon.
Both prefixes are freely used to coin new words, and in that case, you're on your own. Unagile is not quick and not graceful, but what is disagile? Unagile or downright clumsy?
I'd definitely re-cast the sentence and use a current English word instead of inventing one. Sluggishness?
But to address the question, the Anglo-Saxon "un" would originally have applied to actions (verbs, mostly) like "undress" or "undo." The French "dis" would be for separation, splitting -- "disjoint," say. However, over the centuries these distinctions blur and combine.
Ability and Agility are different kinds of concepts. Ability is strictly a binary (yes/no) thing. You are able to do it, or you are unable to do it. You have the ability, or inability to do something. You should not, in good English, have a "low ability" to do something.
Agility is different. You can have it to a greater or lesser degree. Thus you can have a low level of agility, or a high level.