You need to URL quote the query content, for example your attempted/hypothetical query URL:
https://rtcserver/ccm/oslc/contexts/somekey/workitems.json?oslc_cm.query=dcterms:type=task
Has to be encoded for actual use as:
https://rtcserver/ccm/oslc/contexts/somekey/workitems.json?oslc_cm.query=dcterms%3Atype%3Dtask
For more details/background see the OSLC query specification https://open-services.net/bin/view/Main/OSLCCoreSpecQuery - look for the section at the bottom "URL Encoding"
The example there, just for the parameter part of the URL is::
Not encoded:
?oslc.where=dcterms:title="test case 1" and dcterms:modified>="2008-12-02T18:42:30"
Encoded:
?oslc.where=dcterms%3Atitle%3D%22test%20case%201%22%20and%20dc%3Amodified%3E%3D%222008-12-02T18%3A42%3A30%22