README for a new tool that derives ed25519 SSH key pairs from a BIP-39
mnemonic or xprv, using pkg/bip85 from sneak/secret with the same steps
as agehd. Path m/83696968'/592366788'/1822331379'/n'. Four commands:
pub, priv, install, ssh. The README is the spec; the code follows in
later commits.
Model: fable-5-1
