The mnemonic command: child mnemonics (closes #4)

keyfunc mnemonic derives a 12, 18 or 24 word child mnemonic through BIP-85's own mnemonic application, with the specification's test vectors as tests. One review round, passed with no findings; the reviewer reproduced the vectors from an implementation written from the specification alone.

Model: opus-5 (implementation and review); fable-5-1 (landing)

internal/cli/mnemonic/mnemonic.go

@@ -0,0 +1,65 @@
+// Package mnemonic is the command that prints a child mnemonic.
+package mnemonic
+
+import (
+	"fmt"
+
+	"git.eeqj.de/sneak/keyfunc/internal/childmnemonic"
+	"git.eeqj.de/sneak/keyfunc/internal/cli/options"
+	"git.eeqj.de/sneak/keyfunc/internal/derive"
+	"github.com/spf13/cobra"
+)
+
+// Command returns the mnemonic command.
+func Command() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "mnemonic",
+		Short: "print a child mnemonic derived from the main one",
+		Args:  cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			child, err := derived(cmd)
+			if err != nil {
+				return err
+			}
+
+			_, err = fmt.Fprintln(cmd.OutOrStdout(), child)
+			if err != nil {
+				return fmt.Errorf("writing the mnemonic: %w", err)
+			}
+
+			return nil
+		},
+	}
+
+	cmd.Flags().Uint32(
+		"words", childmnemonic.DefaultWords,
+		"how long the child mnemonic is: 12, 18 or 24 words",
+	)
+
+	return cmd
+}
+
+// derived returns the child mnemonic this run asks for.
+func derived(cmd *cobra.Command) (string, error) {
+	index, err := options.Index(cmd)
+	if err != nil {
+		return "", err
+	}
+
+	words, err := cmd.Flags().GetUint32("words")
+	if err != nil {
+		return "", fmt.Errorf("reading the word count: %w", err)
+	}
+
+	parent, err := options.Mnemonic(cmd)
+	if err != nil {
+		return "", err
+	}
+
+	master, err := derive.Master(parent)
+	if err != nil {
+		return "", err
+	}
+
+	return childmnemonic.Derive(master, words, index)
+}