It happens sometimes that users post bases for answers in comments. Those might give some first hints but might not be entirely fleshed out enough to provide a full answer. If you find a comment that you think should be an answer there are indeed various possibilities you listed, let's look at them.
1. Ping the commenter with another comment?
This is indeed a good approach. It gives the commenter a chance to flesh out something that he might not have thought worth an answer. But sometimes users just need a little encouragement that the thoughts they might deem unworthy actually make for good answers. Even 5k users, who should know what a proper answer is might not always be totally conviced about their ideas or might not have the time for writing a complete answer, rather than a little 3-liner. But a user saying "hey, that could be a very good answer" might already be enough to motivate them into writing it up as a proper answer.
Don't worry too much about cluttering up the comment section in this case. Comments that improve the site and its posts are always valuable and are exactly what comments are for. And if that comment leads to a user writing a good answer, then it obviously improves the site. Once the answer is written, those comments can always be flagged as "obsolete" and be cleaned up later.
2. Flag the comment?
There is really no need to involve any moderators on this matter right away. Flagging seems not the right action at all here. As it's also unclear what moderators could even do about the situation.
3. Post the comment as an answer myself (with reference to the commenter)?
Given the commenter does not want to write an answer, this seems to be the best action to do and that is of most use for the site. You can attribute it to the commenter if you want and it's a nice gesture, but not strictly necessary. You might want to precede that by step 1 to give him some chance to answer it himself, but you don't have to either. Afterall it's everyone's own fault if they don't post an answer as an answer and you're not to be blamed for writing a valid answer.
Neither should there be any obligation to make that answer Community Wiki. This is absolutely not what Communtiy Wiki is for. This is still your answer and frankly, if the commenter didn't write it as a proper answer, that's pretty much their fault. You still took the time to do so and flesh it out into a proper answer, especially since you did not just take the comment but went to the additional work of properly elaborating why this is a good answer.
You deserve that reputation and Community Wiki is not for eschewing the concept of reputation for good content on which the entirety of SE is built. Community Wiki is to enable the collaborative construction of an answer, which usually isn't the case when you just grabbed an idea roughly sketched in a comment and fleshed it out into a proper answer. The sole purpose of enabling lower rep users to easily edit an answer is not required in most of those cases at all and there simply is no other use for Community Wiki.
What happened with this specific answer?
Well, what happened here was that I saw an answer that was Community Wiki without seeing any clear reason why that was. I didn't even know this was for the (as explained above, flawed) reason of having been mentioned in a comment before, because, well, the original comment wasn't even mentioned in that answer. If your purpose really was to give the original commenter attirbution for his work, then you should rather have actually mentioned that comment. If that was not your purpose, then there is absolutely no reason for eschewing the reputation from that answer either.
While not seeing any reason for this to be Community Wiki, I still did not immediately undo that state, since I thought there might be another (and the sole valid) reason for it being CW. I thought it might be the attempt at a comprehensive single large collaboratively composed canonical answer. Those might sometimes make sense, as can be seen in this recent example (for now it's only answered by a single user, though, and once it's finished and most of it still came from that user, I'll remove the CW status, too, but in its current form that makes sense).
But once I saw another user posting another good answer, I felt it such a canonical answer was simply not necessary here (as they're also a little iffy sometimes anyway). So that single valid reason for CW vanishing, I turned it into a normal answer. Then I noticed that a comment on the question said pretty much the same thing, so I deleted that comment since it was of course obsolete now.