Require secret at the commit that drops go-bip39 (closes #70)

sneak/secret now carries its own copy of go-bip39
(sneak/secret#130), so requiring it at that
commit, ef0ae90, lets go mod tidy drop go-bip39 from go.sum. Earlier,
secret renamed its module to sneak.berlin/go/secret
(sneak/secret#43), so the requirement, the
two imports of its bip85 package and the README sentence naming that
package move to the new path. The bip85 functions keyfunc calls are
unchanged, and no other requirement moves.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -6,7 +6,7 @@ identities and child mnemonics, each of which can be recreated from that
 mnemonic at any time. The same mnemonic, key type and index always give the same
 key.
 
-It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
+It uses the BIP-85 entropy deriver from `sneak.berlin/go/secret/pkg/bip85` and
 takes the same steps as that repository's `agehd` package.
 
 Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH keys,