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.

A mnemonic command that runs keyfunc itself no longer finds the main
mnemonic in either variable.

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

internal/cli/ssh/to.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/cli/signals"
+	"sneak.berlin/go/keyfunc/internal/mnemonic"
 )
 
 // StatusError says the tool should end with the status ssh ended with.
@@ -79,7 +80,7 @@ func to() *cobra.Command {
 func connect(ctx context.Context, argv []string) error {
 	//nolint:gosec // the arguments are the user's own, meant for ssh
 	command := exec.CommandContext(ctx, "ssh", argv...)
-	command.Env = childEnv()
+	command.Env = mnemonic.ChildEnv()
 	command.Stdin = os.Stdin
 	command.Stdout = os.Stdout
 	command.Stderr = os.Stderr