Say the mnemonic still opens a vault its unlocker cannot (closes #47)

When a vault cannot be opened through its current unlocker, the error now
ends by naming the vault, saying that it still opens with its mnemonic,
and that 'secret unlocker add passphrase' run with SB_SECRET_MNEMONIC set
gives it a new unlocker, after 'secret vault select' when it is not the
current vault. Only when the vault metadata records the key the mnemonic
derives, and not when the passphrase could not be read. 'secret encrypt'
and 'secret decrypt' read the key secret through vault.GetSecret, and
Secret.GetValue with its helpers is removed. An unreadable 'current'
file's error names 'secret version list' and 'secret version promote'.
Causes stay wrapped.

Model: opus-5-5

internal/secret/secret_test.go

@@ -2,7 +2,6 @@
 package secret
 
 import (
-	"encoding/json"
 	"errors"
 	"os"
 	"path/filepath"
@@ -13,7 +12,6 @@ import (
 	"git.eeqj.de/sneak/secret/pkg/agehd"
 	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
-	"github.com/stretchr/testify/require"
 )
 
 // testMnemonicValue is the standard BIP39 test vector mnemonic.
@@ -321,46 +319,3 @@ func TestPerSecretKeyFunctionality(t *testing.T) {
 		t.Logf("Secret.Exists() works correctly")
 	})
 }
-
-// TestSecretGetValueWithMnemonicUsesVaultDerivationIndex checks that
-// GetValue, given the mnemonic, derives the long-term key at the derivation
-// index in the vault's metadata. At index 0 it could not decrypt the secret,
-// which was encrypted to the key at index 1.
-func TestSecretGetValueWithMnemonicUsesVaultDerivationIndex(t *testing.T) {
-	t.Parallel()
-
-	fs := afero.NewMemMapFs()
-	vaultDir := "/test-config/vaults.d/test-vault"
-
-	mnemonic := memguard.NewBufferFromBytes([]byte(testMnemonicValue))
-	defer mnemonic.Destroy()
-
-	vlt := &MockVault{
-		name:            "test-vault",
-		fs:              fs,
-		directory:       vaultDir,
-		derivationIndex: 1,
-		mnemonic:        mnemonic,
-	}
-
-	metadata, err := json.Marshal(VaultMetadata{DerivationIndex: vlt.derivationIndex})
-	require.NoError(t, err)
-	require.NoError(t, fs.MkdirAll(vaultDir, DirPerms))
-
-	err = afero.WriteFile(
-		fs, filepath.Join(vaultDir, "vault-metadata.json"), metadata, FilePerms)
-	require.NoError(t, err)
-
-	secretName, secretValue := "x", "value"
-
-	err = vlt.AddSecret(secretName,
-		memguard.NewBufferFromBytes([]byte(secretValue)), false)
-	require.NoError(t, err)
-
-	value, err := NewSecret(vlt, secretName).GetValue(nil, mnemonic)
-	require.NoError(t, err)
-
-	defer value.Destroy()
-
-	require.Equal(t, secretValue, value.String())
-}