Keep the mnemonic variables from the mnemonic command (closes #80)

The mnemonic command ran with the tool's whole environment, so a
mnemonic set in KEYFUNC_MNEMONIC reached it, although the command is
there to supply one of its own. It now gets the environment ssh and
sftp already get, with KEYFUNC_MNEMONIC and KEYFUNC_MNEMONIC_COMMAND
removed. The function that builds that environment moves from the ssh
command group into internal/mnemonic, beside the two variable names,
so all three programs use the one copy. The README says so.

Model: opus-5-5

internal/mnemonic/mnemonic_test.go

@@ -1,6 +1,8 @@
 package mnemonic_test
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -61,6 +63,31 @@ func TestTheMnemonicInTheEnvironmentComesLast(t *testing.T) {
 	require.Equal(t, fromVariable, words)
 }
 
+func TestTheMnemonicIsNotHandedToTheCommand(t *testing.T) {
+	const marker = "KEYFUNC_TEST_MARKER"
+
+	t.Setenv(mnemonic.CommandVariable, prints(fromCommandVariable))
+	t.Setenv(mnemonic.Variable, fromVariable)
+	t.Setenv(marker, "reaches the command")
+
+	path := filepath.Join(t.TempDir(), "environment")
+
+	words, err := mnemonic.Read(
+		t.Context(), "env > '"+path+"'; "+prints(example()),
+	)
+	require.NoError(t, err)
+	require.Equal(t, example(), words)
+
+	//nolint:gosec // the path is a temporary file of the test's own
+	content, err := os.ReadFile(path)
+	require.NoError(t, err)
+
+	environment := string(content)
+	require.NotContains(t, environment, mnemonic.Variable+"=")
+	require.NotContains(t, environment, mnemonic.CommandVariable+"=")
+	require.Contains(t, environment, marker+"=")
+}
+
 func TestAFailingCommandSaysWhatWentWrong(t *testing.T) {
 	t.Setenv(mnemonic.CommandVariable, "")
 	t.Setenv(mnemonic.Variable, "")