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/version.go

@@ -23,7 +23,6 @@ const (
 
 // Sentinel errors for version operations
 var (
-	errVersionNotFound            = errors.New("not found for secret")
 	errCannotRemoveCurrentVersion = errors.New("promote another version first")
 )
 
@@ -156,7 +155,7 @@ func (cli *Instance) ListVersions(cmd *cobra.Command, secretName string) error {
 	if !exists {
 		secret.Debug("Secret not found", "secret_name", secretName)
 
-		return fmt.Errorf("secret '%s' %w", secretName, errSecretNotFound)
+		return fmt.Errorf("secret '%s' %w", secretName, vault.ErrSecretNotFound)
 	}
 
 	// List all versions
@@ -289,7 +288,7 @@ func (cli *Instance) PromoteVersion(
 
 	if !exists {
 		return fmt.Errorf("version '%s' %w '%s'",
-			version, errVersionNotFound, secretName)
+			version, vault.ErrVersionNotFound, secretName)
 	}
 
 	// Update the current symlink using the proper function
@@ -374,7 +373,7 @@ func (cli *Instance) findVersionToRemove(
 
 	if !exists {
 		return versionToRemove{},
-			fmt.Errorf("secret '%s' %w", secretName, errSecretNotFound)
+			fmt.Errorf("secret '%s' %w", secretName, vault.ErrSecretNotFound)
 	}
 
 	// Check if version exists
@@ -386,7 +385,7 @@ func (cli *Instance) findVersionToRemove(
 
 	if !exists {
 		return versionToRemove{}, fmt.Errorf("version '%s' %w '%s'",
-			version, errVersionNotFound, secretName)
+			version, vault.ErrVersionNotFound, secretName)
 	}
 
 	// Get current version