Keep the mnemonic out of the ssh and sftp children (closes #16)

ssh to and ssh install started the system ssh and sftp with the tool's
whole environment, so a mnemonic taken from KEYFUNC_MNEMONIC stayed in
the child's environment for as long as it ran, readable by the same user
and forwardable to the host through a SendEnv line. ssh to keeps the
private key inside the tool; the mnemonic must not leave it either.

A shared helper in the ssh cli package hands both children the tool's
environment with KEYFUNC_MNEMONIC and KEYFUNC_MNEMONIC_COMMAND removed.
The mnemonic command itself still runs with the full environment. The
stand-in ssh and sftp in the tests now record their environment, and two
tests show neither variable reaches them while another one does.

Model: opus-4-8

internal/cli/ssh/ssh.go

@@ -3,9 +3,12 @@ package ssh
 
 import (
 	"fmt"
+	"os"
+	"strings"
 
 	"git.eeqj.de/sneak/keyfunc/internal/cli/options"
 	"git.eeqj.de/sneak/keyfunc/internal/derive"
+	"git.eeqj.de/sneak/keyfunc/internal/mnemonic"
 	"git.eeqj.de/sneak/keyfunc/internal/sshkey"
 	"github.com/spf13/cobra"
 )
@@ -84,6 +87,26 @@ func write(cmd *cobra.Command, text string) error {
 	return nil
 }
 
+// childEnv is the tool's environment with the mnemonic variables taken
+// out, for the ssh and sftp children it starts. "ssh to" exists so the
+// private key never leaves the tool; the mnemonic, from either variable,
+// must not leave it either.
+func childEnv() []string {
+	environ := os.Environ()
+	kept := make([]string, 0, len(environ))
+
+	for _, entry := range environ {
+		name, _, _ := strings.Cut(entry, "=")
+		if name == mnemonic.Variable || name == mnemonic.CommandVariable {
+			continue
+		}
+
+		kept = append(kept, entry)
+	}
+
+	return kept
+}
+
 // addComment gives a command its comment flag.
 func addComment(cmd *cobra.Command) {
 	cmd.Flags().String(