Tonight when I noted that Pickles was downstairs, I quietly shut the door to the upstairs. For a moment she was alarmed but, as I'd hoped, she found downstairs places to hide. Then she snuggled in with Fluffy while I worked only five feet away.
When I opened the upstairs door again, she remained downstairs for quite awhile and even let me walk back and forth in front of her chair without bolting.
I'll count "victory" the first night she jumps up to sleep on the bed with Fluff, when I'm in it. She did discover the catio, and thinks outdoors is pretty amazing.