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

`keyfunc ssh to` and `keyfunc ssh install` started the system `ssh` and `sftp` with the tool's whole environment, so a mnemonic given in `KEYFUNC_MNEMONIC` stayed readable in the child's environment and could be forwarded to the host by a `SendEnv` line. Both children now get the environment with `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` removed, through one helper, `childEnv`, in the ssh cli package. The mnemonic command still runs with the full environment. Two tests drive the real commands against the stand-in `ssh` and `sftp` and check that a third variable still arrives.

Model: opus-4-8 (implementation, review); fable-5-1 (merge message)

internal/cli/ssh/install.go

@@ -166,6 +166,7 @@ func session(
 
 	//nolint:gosec // the options are the user's own, meant for sftp
 	command := exec.CommandContext(cmd.Context(), "sftp", argv...)
+	command.Env = childEnv()
 	command.Stdin = strings.NewReader(strings.Join(batch, "\n") + "\n")
 	command.Stdout = &said
 	command.Stderr = &said