sshserver: add scp support

fixes https://github.com/boldsoftware/sketch/issues/53
diff --git a/go.mod b/go.mod
index ccadaf1..a9ea492 100644
--- a/go.mod
+++ b/go.mod
@@ -22,9 +22,11 @@
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/gliderlabs/ssh v0.3.8 // indirect
 	github.com/kevinburke/ssh_config v1.2.0 // indirect
+	github.com/kr/fs v0.1.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/oklog/ulid/v2 v2.1.0 // indirect
+	github.com/pkg/sftp v1.13.9 // indirect
 	github.com/sashabaranov/go-openai v1.38.2 // indirect
 	golang.org/x/mod v0.24.0 // indirect
 	golang.org/x/sys v0.32.0 // indirect