What is the exact equivalent to “daylight ghost’?

I happen to have read that story. The narrator is a woman who is currently not in the work force and is reassessing everything about her life. On the shallower end of the pool, the story is a nearly morbid bout of psychological navel-gazing; at the deeper end, it calls into question how the artifice of the ego is constructed by outside forces just as much as from those inside.

I say that to preface this thought: the daylight ghost is meant to illuminate someone who is not really there, existing in a half-world, without real purpose and so without real existence. As Mark Beadles says, this is not an idiom (though I might quibble whether daylight served as an adjective there instead of a noun—but it hardly matters). The term is invented for the story, and is an example of how description can be rich while also being directly to the point. This isn't a lot of flowery adjectives: the descriptive terms are bullets in a gun that the narrator is aiming right at her own head.

I think here it is used precisely because it is not a common idiom. We can get the precise meaning from the rest of the passage, and hence the risk of it not being understood is reduced.

In writing generally, we want to have some imagery that is fresh and novel. Indeed, the familiar idioms start as such, then some are borrowed often. This tends to result in clichés that are boring and tired because they have been used so much that the original freshness is gone, and then they either die out or become idioms - where once they were used because they startled, now they are used because they have a simple meaning that is immediately understood.

The strength of the expression tells us a lot about what the narrator thinks of her situation, and the choice of words must be understood in that context. Certainly you should not use housewife this way generally - it's one thing to describe the perspective of a narrator who can only consider it negatively, but to use it that way generally could be seen as very insulting to homemakers.

Of course daylight ghost is not a 'word'; it's also not a construction nor an idiom. It's just two nouns in apposition, with daylight modifying ghost: a ghost of the daylight. The sense is that this is in contrast to the usual ghost which haunts the night.

You will not find it in a dictionary for the same reason you will not find "kangaroo food": the meaning of the phrase is understood from its parts.