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. They are now string flags read by options.Decimal, which
takes decimal digits only: a leading zero stays decimal, and a base
prefix, an underscore or a sign is refused with an error naming the
flag, before the mnemonic is looked for. --help shows both as a
number, and the README says the index is a decimal number.

Model: opus-5-5

internal/cli/mnemonic/mnemonic.go

@@ -3,6 +3,7 @@ package mnemonic
 
 import (
 	"fmt"
+	"strconv"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/childmnemonic"
@@ -31,9 +32,9 @@ func Command() *cobra.Command {
 		},
 	}
 
-	cmd.Flags().Uint32(
-		"words", childmnemonic.DefaultWords,
-		"how long the child mnemonic is: 12, 18 or 24 words",
+	cmd.Flags().String(
+		"words", strconv.Itoa(childmnemonic.DefaultWords),
+		"the `number` of words in the child mnemonic: 12, 18 or 24",
 	)
 
 	return cmd
@@ -46,9 +47,9 @@ func derived(cmd *cobra.Command) (string, error) {
 		return "", err
 	}
 
-	words, err := cmd.Flags().GetUint32("words")
+	words, err := options.Decimal(cmd, "words")
 	if err != nil {
-		return "", fmt.Errorf("reading the word count: %w", err)
+		return "", err
 	}
 
 	parent, err := options.Mnemonic(cmd)