Per sneak: no xprv input. The mnemonic comes from a shell command given
as a flag or an environment variable (for example `secret get foo`),
from an environment variable holding it, or from a no-echo prompt. The
file flag is dropped since the command covers it.
Model: fable-5-1
The tool will cover more key types than SSH, so the directory, module
and binary become bip85keys and the commands are grouped by type:
"bip85keys ssh pub|priv|install|ssh" now, "bip85keys age pub|priv"
planned. The README states the age derivation (agehd's path in
sneak/secret) and that adding a type is one package plus one
subcommand. The SSH application id and everything else stay as before.
Model: fable-5-1
