fix: resolve mnd (magic number) linter errors in agehd and bip85 packages

- Define x25519KeySize constant (32) at package level in agehd
- Replace all magic number 32 uses with x25519KeySize constant
- Define bech32BitSize8 and bech32BitSize5 constants for bit conversions
- Define bip85EntropySize constant (64) for entropy validation
- Define BIP39 word count constants (words12-24) with descriptive names

pkg/bip85/bip85.go

@@ -52,8 +52,9 @@ type DRNG struct {
 
 // NewBIP85DRNG creates a new DRNG seeded with BIP85 entropy
 func NewBIP85DRNG(entropy []byte) *DRNG {
+	const bip85EntropySize = 64 // 512 bits
 	// The entropy must be exactly 64 bytes (512 bits)
-	if len(entropy) != 64 {
+	if len(entropy) != bip85EntropySize {
 		panic("DRNG entropy must be 64 bytes")
 	}
 
@@ -169,17 +170,26 @@ func DeriveBIP39Entropy(masterKey *hdkeychain.ExtendedKey, language, words, inde
 	}
 
 	// Determine how many bits of entropy to use based on the words
+	// BIP39 defines specific word counts and their corresponding entropy bits
+	const (
+		words12 = 12 // 128 bits of entropy
+		words15 = 15 // 160 bits of entropy
+		words18 = 18 // 192 bits of entropy
+		words21 = 21 // 224 bits of entropy
+		words24 = 24 // 256 bits of entropy
+	)
+	
 	var bits int
 	switch words {
-	case 12:
+	case words12:
 		bits = 128
-	case 15:
+	case words15:
 		bits = 160
-	case 18:
+	case words18:
 		bits = 192
-	case 21:
+	case words21:
 		bits = 224
-	case 24:
+	case words24:
 		bits = 256
 	default:
 		return nil, fmt.Errorf("invalid BIP39 word count: %d", words)