go-bip39 gone from go.mod and go.sum (#72)

What is on `next`, reviewed.

- go-bip39 is gone from `go.mod` and `go.sum`: keyfunc now requires `sneak/secret` at `ef0ae90`, the commit that gives `secret` its own copy (sneak/secret#130). No derived key, mnemonic or test vector changes.

To know before merging:

- `secret`'s module is now `sneak.berlin/go/secret` (sneak/secret#43), so keyfunc's requirement, its two imports of `secret`'s `bip85` package and one README sentence use that path. The Go module proxy serves that path, so `go install` keeps working.

Model: opus-5-5
Reviewed-on: #72
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

go.mod

@@ -4,13 +4,13 @@ go 1.26.0
 
 require (
 	filippo.io/age v1.3.2
-	git.eeqj.de/sneak/secret v0.0.0-20260810132333-41cea400a7fd
 	github.com/btcsuite/btcd v0.25.0
 	github.com/btcsuite/btcd/btcutil v1.2.0
 	github.com/spf13/cobra v1.10.2
 	github.com/stretchr/testify v1.12.1
 	golang.org/x/crypto v0.57.0
 	golang.org/x/term v0.46.0
+	sneak.berlin/go/secret v0.0.0-20261007105610-ef0ae90768c8
 )
 
 require (