When trying to add an ecdsa-sk key to gpg-agent via `ssh-add`, I'm getting an error:
```
$ ssh-add ~/.ssh/id_ecdsa_sk
Could not add identity "/Users/sven/.ssh/id_ecdsa_sk": agent refused operation
```
With `debug-level advanced`, the log shows
```
2021-06-18 19:44:19 gpg-agent[43421] ssh handler 0x70000c035000 for fd 8 started
2021-06-18 19:44:19 gpg-agent[43421] ssh request handler for add_identity (25) started
2021-06-18 19:44:19 gpg-agent[43421] ssh request handler for add_identity (25) ready
2021-06-18 19:44:19 gpg-agent[43421] ssh handler 0x70000c035000 for fd 8 terminated
```
I couldn't find a ticket for this. If this is a duplicate, feel free to close the ticket.