Please link me to documentation on how to use this for all writing queries we do now. Specifically
We do some others as well but they probably don't need any tagging (action=unwatchlist and so on)
You need to create the tag using Special:Tags on each wiki where it is needed, or ask an administrator to do it for you.
Does the edit still succeed when this warning comes back? I can't remember how I implemented it...
OK, the feature is probably working, given that admin create this tag and insert tag:name_of_tag in /Config
You will see this tag however only for about 50% of edits made by huggle as most of these are rollbacks which don't support it.
In Special:Tags in enwiki it says the huggle tag is "Defined by an extension". If you're a sysop there you need to use the interface on Special:Tags to manually create the tag. Otherwise request such an action from a sysop. AbuseFilter tags can only be applied by AbuseFilter, so that won't cut it.
The current huggle tag is "defined by an extension" because it was created through the edit filter. I've created a tag (Huggle as opposed to huggle, see the bottom of Special:Tags) which can be included in the tool, presumably in the same way you were trying before.
I think it would be better for consistency if tag was in lowercase as other tags. If it can't be created now as it conflicts with existing one, it should be probably added once the edit filter is disabled. But that shouldn't happen before 3.1.17 is released.