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

README.md

@@ -54,6 +54,11 @@ If none of these is available and standard input is not a terminal, the tool
 refuses and exits with status 1. A mnemonic that fails the BIP-39 checksum is
 refused with a message saying so.
 
+`KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
+environment before the system `ssh` (`keyfunc ssh to`) and `sftp`
+(`keyfunc ssh install`) are started, so the mnemonic is never handed on to
+them.
+
 Every command takes `--index` / `-n` and `--mnemonic-command`, and has `--help`.
 `keyfunc --version` prints the version. `make build` stamps it; a binary
 installed with `go install` reports the module version instead.