A wrong passphrase gives an error that says so (closes #157)

A passphrase that did not decrypt the passphrase unlocker failed with age's
"no identity matched any of the recipients" and advice to give the vault a
new unlocker from its mnemonic, which would replace an unlocker that works.
GetIdentity now turns that age error into "wrong passphrase for unlocker
<id>", wrapping the new secret.ErrWrongPassphrase, and withMnemonicAdvice
leaves it alone, as it does when no passphrase was read. A passphrase from
SB_UNLOCK_PASSPHRASE and a typed one both reach this code. A priv.age that
is not an age file keeps the advice.

Judgement call: age cannot tell a wrong passphrase from a damaged key inside
a priv.age whose header reads, so that case now reads as a wrong passphrase.

Model: opus-5-5

internal/vault/vault.go

@@ -291,10 +291,13 @@ func (v *Vault) unlockLongTermKey(
 // without a mnemonic records none, and without its metadata the key cannot
 // be derived. It is not added when the passphrase could not be read: the
 // unlocker was not tried, and adding one would need a passphrase read the
-// same way. Nor is it added when the current unlocker does not exist and the
-// vault has others: the error already says to select one of them.
+// same way. Nor is it added when the passphrase is wrong: the unlocker still
+// opens the vault with the right one. Nor is it added when the current
+// unlocker does not exist and the vault has others: the error already says
+// to select one of them.
 func (v *Vault) withMnemonicAdvice(err error) error {
 	if errors.Is(err, secret.ErrPassphraseNotRead) ||
+		errors.Is(err, secret.ErrWrongPassphrase) ||
 		errors.Is(err, ErrUnlockerNotFound) {
 		return err
 	}

internal/vault/vault_error_test.go

@@ -5,7 +5,6 @@ import (
 	"path/filepath"
 	"testing"
 
-	"filippo.io/age"
 	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
@@ -140,9 +139,7 @@ func TestAddSecretForceWithWrongPassphraseChangesNothing(t *testing.T) {
 
 	err = vlt.AddSecret(testSecretName, second, true)
 
-	var noMatch *age.NoIdentityMatchError
-
-	require.ErrorAs(t, err, &noMatch)
+	require.ErrorIs(t, err, secret.ErrWrongPassphrase)
 	assert.Equal(t, before, dirContents(t, fs, secretDir))
 }