No such key mountdir

```
$ keybase config get -b mountdir
▶ ERROR .mountdir: no such key: mountdir
$ ls /Volumes/Keybase
private public team
```

I'm confused by the above. It appears that keybase has mounted okay, but isn't aware of that.

my log id: c3aa7f085ded35786f1e241c

Reproduce `keybase config get -b mountdir` and compare its result with the mounted `/Volumes/Keybase` contents. Trace the configuration lookup and mount-status behavior to determine whether the missing key is expected or indicates a bug; done means explaining or correcting the discrepancy and verifying the command and mount output.

Written by the indexing model from the issue text.