Update. I decided to do a test drive this evening, and lo and behold, the clutch worked just fine. It's been about 4 days to the hour since I last drove it, so it does seem like letting it sit for more than a week at a time has something to do with the problem. The car had been sitting for more than a week when I had my last two drives (before this evening) where the problem showed up.
So I don't have much to go on, although I suspect one fo the following:
1/ Moisture is somehow causing a problem when it sits for a longish time. And it has been very damp out lately.
2/ Resin from the clutch disc is on the flywheel or pressure plate or both, and in cold weather, it tends to make the clutch grab. And it has been cold out lately, although not so much today.
3/ A mouse crawled into the bellhousing through the hole where the clutch fork exits and got ground up, thus contaminating the disc. And I do have mice in my garage (although I think this unlikely and probably shouldn't give it any consideration...LOL)