Copy go-bip39 into internal/bip39 (closes #122)

go-bip39's repository no longer exists. The part of v1.1.0 that secret
uses (NewEntropy, NewMnemonic, NewSeed, IsMnemonicValid and what they
call; English word list only) now lives in internal/bip39 with
upstream's LICENSE beside it, along with upstream's tests of that code
and their vectors unchanged. Every import moves there, and the module
leaves go.mod and go.sum. Beyond the trimming, only what the linter
asked for changed. No derived key or mnemonic changes.

Model: opus-5-5

pkg/agehd/agehd.go

@@ -17,7 +17,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil/hdkeychain"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcutil/bech32"
-	"github.com/tyler-smith/go-bip39"
+	"sneak.berlin/go/secret/internal/bip39"
 	"sneak.berlin/go/secret/pkg/bip85"
 )
 

pkg/agehd/agehd_test.go

@@ -11,7 +11,7 @@ import (
 	"testing"
 
 	"filippo.io/age"
-	"github.com/tyler-smith/go-bip39"
+	"sneak.berlin/go/secret/internal/bip39"
 )
 
 //nolint:dupword // BIP39 test mnemonics repeat words by design

pkg/bip85/bip85_test.go

@@ -10,7 +10,7 @@ import (
 	"testing"
 
 	"github.com/btcsuite/btcd/btcutil/hdkeychain"
-	"github.com/tyler-smith/go-bip39"
+	"sneak.berlin/go/secret/internal/bip39"
 	"sneak.berlin/go/secret/pkg/bip85"
 )