Is “harebrain” used commonly as a verb or adjective?

Solution 1:

Harebrained is more often written as hare-brained, which makes it clearer that it doesn’t come from a root *harebrain, but is analogous to compounds like red-haired, duck-billed, flat-topped and so on.

So, it’s describing what kind of brain someone has: the brain of a hare, a proverbially daft, silly, jumpy animal. Or, by extension, it describes something that a hare-brained person might think up, or do, or support. A more common approximate synonym would be idiotic.

Hare-brained scheme in particular has become almost a cliché. Your example is very similar: “The agreement […] is hare-brained,” is saying it’s a stupid, idiotic idea.

Solution 2:

It's an adjective, modifying "agreement." More commonly, though, it's used in a phrase such as "a harebrained idea."

The meaning, as you've probably gathered from that list of synonyms, is something like "ridiculous" or "nonsensical." So the editorialist is saying that it's a dumb idea.

Solution 3:

The reason is that hares (ie turbocharged Rabbits) run around in the spring mating season, fighting with each other and generally behaving in an undignified manner.

So "mad as a march hare", "haring around", "harebrained" all mean something like frivolous, un-focussed, scatterbrained etc rather than directly stupid.

Solution 4:

Harebrained is an adjective, often describing ideas/schemes/plans that are at least slightly, if not completely, crazy.

I've never heard of the word "harebrain", whether a verb or something else.

Solution 5:

I agree with PLL et al., but I think part of your question is really about confusion regarding the part of speech, which I don't see addressed or named here elsewhere.

Is harebrain used as a verb in passive form or an adjective in the above text? Is it common to use this word, which I thought to be a noun, in this way?

In the cited text, "harebrained" has the appearance of a past participle, which is a verb form used as an adjective, as in "a frightened monkey" (where "frighten" is ordinarily a basic verb); however, I believe in this instance it is a simple adjective, and not a participle. Rephrased, the cited text might read, "...a harebrained agreement". For the word "harebrained", I believe this is typical usage, per PLL's comments.