ssh install refuses stray arguments before -- and a symlinked authorized_keys (closes #61)

ssh install now takes the host alone before --: any other word there, or a second argument without --, is refused before the mnemonic is read or sftp runs, so keyfunc ssh install alice@host frank@host no longer installs the key for frank@host. The first listing of ~/.ssh is now ls -n, which shows the file type, so a symlinked authorized_keys is refused before any upload instead of being replaced by a regular file; the README says so.

Judgement calls: install -- host is refused; a symlinked authorized_keys is refused even when its target already holds the key.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/cli/ssh/install_test.go

@@ -10,7 +10,7 @@ import "testing"
 const (
 	echoed = `sftp> get .ssh/authorized_keys "/tmp/keyfunc/authorized_keys"
 `
-	listed  = "sftp> ls -1 .ssh\n"
+	listed  = "sftp> ls -n .ssh\n"
 	warning = `Warning: Identity file /gone not accessible: ` +
 		"No such file or directory.\n"
 )