With GNOME 46, the provider of the GNOME SSH integration was moved into its own separate package. While new installations will have this installed automatically, old installations will in most cases manually need to install this package:
$ sudo apk add gcr-ssh-agent
This applies to Phosh, GNOME, and GNOME Mobile. Note that installing gcr-ssh-agent automatically makes it the default SSH agent on your system.
Related MR: aports!63893