ssh install tells a missing .ssh from one it cannot enter (closes #10)

The first sftp session now lists .ssh before fetching authorized_keys. The file reads as empty only when sftp reports .ssh itself as missing, or the listing succeeded and the file is reported missing. A directory or file that is there but cannot be read fails the run and nothing is written, so no existing authorized_keys is replaced by content that was not built from what was read. An .ssh that already exists keeps its mode; the directory is made and set to 0700 only when none was found. The README describes the rule and states batch mode's limit: a key or an agent must authenticate.

Model: opus-4-8 (implementation); fable-5-1 (summary)

internal/cli/ssh/install_test.go

@@ -10,6 +10,7 @@ import "testing"
 const (
 	echoed = `sftp> get .ssh/authorized_keys "/tmp/keyfunc/authorized_keys"
 `
+	listed  = "sftp> ls -1 .ssh\n"
 	warning = `Warning: Identity file /gone not accessible: ` +
 		"No such file or directory.\n"
 )
@@ -76,3 +77,57 @@ func TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys(t *testing.T) {
 		})
 	}
 }
+
+// TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo holds the
+// wordings the OpenSSH client was seen to use when a listing fails: a
+// directory it cannot find is reported one way, and one it cannot enter
+// another, and only the first is read as a host with no .ssh yet.
+func TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo(t *testing.T) {
+	t.Parallel()
+
+	listings := map[string]struct {
+		said string
+		want bool
+	}{
+		"the directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there, named as it was asked for": {
+			said: listed + `Can't ls: ".ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there and an identity file is not either": {
+			said: warning + listed +
+				`Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is there and cannot be entered": {
+			said: listed +
+				`remote readdir("/home/someone/.ssh/"): Permission denied` + "\n",
+			want: false,
+		},
+		"some other directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.config" 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, listing := range listings {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			if directoryAbsent(listing.said) != listing.want {
+				t.Errorf(
+					"read as absent: %t, wanted %t, from:\n%s",
+					!listing.want, listing.want, listing.said,
+				)
+			}
+		})
+	}
+}