In my opinion, the tags clustered around grammar and grammaticality have gotten out of hand. All these tags: grammar, grammaticality, grammaticality-in-context, grammatical mean basically the same thing, no?
How should we clean up this situation?
I'd scrap 'em all.
Some people use grammar and its derivatives to designate matters of morphology and concord. This was the most common use among English professors when I was young.
Some people use them to designate sentential well-formedness: morphology-and-syntax, as distinct from phonology on one side and discourse semantics on the other. This appears be the most common use among English professors today.
Some people use them to designate whatever is acceptable to a more or (usually) less well informed 'authority' on 'usage'. This appears to be the most common use among native-speaker laypeople.
Some people use them in more or less the sense of Chomsky's Syntactic Structures, equivalent to "acceptable to a native speaker". This appears to be the most common use among our non-native-speaker visitors.
It seems to me that these terms have no consistent meaning, certainly not a meaning which makes it useful for searching or for informing users what a question is about.
Replace every tag with grammar. No one here makes any practical distinctions between any of these tags, and as far as I can tell, it's unlikely that future users are going to start.
At a minimum, boiling these tags down to the two ELU uses is preferable to keeping the four we have now. But if you want to keep two tags, vote for 200_success's answer rather than this one; this answer is about keeping only one tag.
I propose the synonym:
English.SE has separate tags for [grammar] and [grammaticality], so perhaps we should maintain that distinction here too. (Consistency with English.SE, even if their tag taxonomy is not ideal, would at least facilitate migration of questions between sites.)