fix: resolve exported type stuttering issues (revive)

- Rename VaultMetadata to Metadata in internal/vault package to avoid stuttering
- Rename BIP85DRNG to DRNG in pkg/bip85 package to avoid stuttering
- Update all references in code and tests

pkg/agehd/agehd.go

@@ -54,6 +54,7 @@ func IdentityFromEntropy(ent []byte) (*age.X25519Identity, error) {
 	if err != nil {
 		return nil, fmt.Errorf("bech32 encode: %w", err)
 	}
+
 	return age.ParseX25519Identity(strings.ToUpper(s))
 }
 
@@ -125,6 +126,7 @@ func DeriveIdentity(mnemonic string, n uint32) (*age.X25519Identity, error) {
 	if err != nil {
 		return nil, err
 	}
+
 	return IdentityFromEntropy(ent)
 }
 
@@ -135,5 +137,6 @@ func DeriveIdentityFromXPRV(xprv string, n uint32) (*age.X25519Identity, error)
 	if err != nil {
 		return nil, err
 	}
+
 	return IdentityFromEntropy(ent)
 }