Give each failure one error value (closes #113)

internal/cli's copies of vault.ErrSecretNotFound, ErrVaultNotFound,
ErrVersionNotFound and ErrSecretExists are removed; the commands wrap the
vault errors. Every error of secret.ReadPassphrase wraps
ErrPassphraseNotRead, so its callers no longer add those words.
ResolveGPGKeyFingerprint returns ErrGPGKeyNotFound for a key the keyring
lacks, recognised by gpg's status line. storeInKeychain returns
errNilDataBuffer. bip85's ErrPasswordTooShort and ErrEncodedTooShort go
with their unreachable checks. Tests that matched these errors' text use
errors.Is.

Model: opus-5-5

internal/cli/vault.go

@@ -250,7 +250,7 @@ func (cli *Instance) resolvePassphrase() (*memguard.LockedBuffer, func(), error)
 	// Use secure passphrase input with confirmation
 	passphraseBuffer, err := readSecurePassphrase("Enter passphrase for unlocker: ")
 	if err != nil {
-		return nil, nil, fmt.Errorf("failed to read passphrase: %w", err)
+		return nil, nil, err
 	}
 
 	return passphraseBuffer, passphraseBuffer.Destroy, nil
@@ -353,7 +353,7 @@ func (cli *Instance) vaultImportPreflight(
 
 	if !exists {
 		return "", "", "", fmt.Errorf("vault '%s' %w",
-			vaultName, errVaultDoesNotExist)
+			vaultName, vault.ErrVaultNotFound)
 	}
 
 	// Check if vault already has a public key
@@ -644,7 +644,7 @@ func (cli *Instance) findVaultToRemove(name string) (vaultToRemove, error) {
 
 	if !slices.Contains(vaults, name) {
 		return vaultToRemove{},
-			fmt.Errorf("vault '%s' %w", name, errVaultDoesNotExist)
+			fmt.Errorf("vault '%s' %w", name, vault.ErrVaultNotFound)
 	}
 
 	if len(vaults) == 1 {