README child-mnemonic vector and host-key note; ssh install refuses a ~/.ssh it cannot enter (closes #51)

The README gains the child mnemonic that `keyfunc mnemonic` prints for
the abandon ... about mnemonic at index 0, asserted by the README
vectors test, and says that `ssh install` needs the host key already
known, with the two ways round it.

`ssh install` now also lists `.ssh/.` before fetching. A `.ssh` that can
be read but not entered lists as empty, so the fetch read it as holding
no file and tried an upload that could only fail. The listing of
`.ssh/.` fails instead, and the tool refuses before any upload, saying
the directory cannot be entered. The test that failed the put with a
file where `.ssh` belongs now uses a `.ssh` of mode 500.

Model: opus-5-5

internal/cli/cli_test.go

@@ -22,6 +22,11 @@ const (
 		"0I4FKs+eVUulTPHfk9VtXw1tMF"
 )
 
+// The child mnemonic the README says the example mnemonic gives at
+// index 0.
+const childZero = "prosper short ramp prepare exchange stove life " +
+	"snack client enough purpose fold"
+
 // The two child mnemonic lengths the tests ask for.
 const (
 	twelve     = 12
@@ -45,6 +50,9 @@ func TestTheReadmeTestVectors(t *testing.T) {
 		vectorOne+" keyfunc/ssh/1",
 		strings.TrimSpace(run(t, "ssh", "pub", "-n", "1")),
 	)
+	require.Equal(t,
+		childZero, strings.TrimSpace(run(t, "mnemonic", "-n", "0")),
+	)
 }
 
 func TestTheCommentCanBeChosen(t *testing.T) {