Do you (or anyone, really) know of a place with a decent definition of how AGPL code needs to be treated?

I'm not clear about what the line is that says "everything from this point must have its source released" versus "all this stuff is OK to license as you please"

Sort of. Thanks to section 13 in each license, AGPLv3 and GPLv3 code can be combined. You end up keeping the Affero provisions on your work, which should be sufficient protection.