Read --index and --words as decimal numbers (closes #75)

Both flags were uint32 flags, which the flag library reads with Go's
base-0 rules, so `-n 010` derived key 8 and `0x10` or `1_0` were
accepted. Both now hold an options.Decimal, a flag value whose Set
parses with strconv.ParseUint(text, 10, 32): a leading zero stays
decimal, and a base prefix, an underscore, a sign or an empty value
is refused with an error naming the flag while the flags are parsed,
before any command runs. Its type is still named uint32, so --help is
unchanged and GetUint32 still reads it back. The README says the index
is a decimal number.

Model: opus-5-5

internal/cli/mnemonic/mnemonic.go

@@ -31,8 +31,9 @@ func Command() *cobra.Command {
 		},
 	}
 
-	cmd.Flags().Uint32(
-		"words", childmnemonic.DefaultWords,
+	words := options.Decimal(childmnemonic.DefaultWords)
+	cmd.Flags().Var(
+		&words, "words",
 		"how long the child mnemonic is: 12, 18 or 24 words",
 	)