It's not really a syncing protocol - a syncing protocol is something more like FeedSync (formally SSE)[1].
This looks more like an API for a feed reader type application. From a quick read it looks mostly ok for that (although I'd question why...)
Out of interest, why not implement a read-only version of rfc5023/AtomPub[2]?
Anyway...
I'd suggest you get rid of the "Article" Object, and use the Atom "Entry" object instead (or possibly a single-entry Atom feed if you are in XML mode), serialized as JSON
The "Fetch" call should probably take a "since date" as an option, instead of just "since id". It should return a JSON serialized Atom feed, not a custom object.
https://cwiki.apache.org/confluence/display/ABDERA/JSON+Seri... is a pretty decent JSON serialization spec for Atom
Notable omissions: no concept of tagging or saving articles.