The ssh install command works over sftp (closes #10)

The command no longer sends a shell script to the host. It fetches
~/.ssh/authorized_keys with the system sftp in batch mode, adds the key
line here, and writes the file back in a second session: mkdir and chmod
on ~/.ssh, put to authorized_keys.keyfunc-<random>, chmod 600, rename
over authorized_keys. Adding the line connects twice.

The file reads as empty only when sftp reported it as not there, in the
line naming that path; the same wording elsewhere -- ssh writes it about
an identity file it cannot find -- does not count, so a file that cannot
be read is never written over. A failed step removes nothing and names
the uploaded file once sftp's echo shows the put was reached.

Model: opus-5

internal/cli/ssh/install_test.go

@@ -0,0 +1,78 @@
+//nolint:testpackage // absent is what these wordings are read by
+package ssh
+
+import "testing"
+
+// What a session says besides its report on the file that was asked
+// for: sftp echoes the command it is running, and ssh warns about an
+// identity file it cannot find in the words of a missing file even
+// though the session goes on to authenticate.
+const (
+	echoed = `sftp> get .ssh/authorized_keys "/tmp/keyfunc/authorized_keys"
+`
+	warning = `Warning: Identity file /gone not accessible: ` +
+		"No such file or directory.\n"
+)
+
+// TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys holds the
+// wordings the OpenSSH client was seen to use against a real server:
+// a file it cannot find is reported one way, naming the path the
+// server expanded, and everything else it says is a failure.
+func TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys(t *testing.T) {
+	t.Parallel()
+
+	sessions := map[string]struct {
+		said string
+		want bool
+	}{
+		"the file is not there": {
+			said: echoed +
+				`File "/home/someone/.ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is not there, named as it was asked for": {
+			said: echoed + `File ".ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is not there and an identity file is not either": {
+			said: warning + echoed +
+				`File "/home/someone/.ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is there and cannot be read": {
+			said: echoed +
+				`remote open "/home/someone/.ssh/authorized_keys": ` +
+				"Permission denied\n",
+			want: false,
+		},
+		"only an identity file is not there": {
+			said: warning + echoed +
+				`remote open "/home/someone/.ssh/authorized_keys": ` +
+				"Permission denied\n",
+			want: false,
+		},
+		"some other file is not there": {
+			said: echoed + `File "/home/someone/.ssh/known_hosts" not found.` +
+				"\n",
+			want: false,
+		},
+		"the connection did not come up": {
+			said: "ssh: connect to host example.com port 22: " +
+				"Connection refused\nConnection closed\n",
+			want: false,
+		},
+	}
+
+	for name, session := range sessions {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			if absent(session.said) != session.want {
+				t.Errorf(
+					"read as absent: %t, wanted %t, from:\n%s",
+					!session.want, session.want, session.said,
+				)
+			}
+		})
+	}
+}