Check arguments before looking for the mnemonic (closes #79)

The key index, --words, the --to recipients and the input file of
age encrypt and age decrypt were checked only after the mnemonic had
been obtained, so a run that could not succeed still prompted for it or
ran the mnemonic command. options.Index now refuses an index above
derive.MaxIndex, which covers every command group; the mnemonic command
calls the new childmnemonic.CheckWords first; age encrypt reads its
recipients with the new agekey.Recipients, and both age commands open
their input, before the key is derived. The README says which checks
come first.

Judgement call: the -o path is still looked at after the mnemonic.

Model: opus-5-5

internal/childmnemonic/childmnemonic.go

@@ -29,6 +29,17 @@ const DefaultWords = twelve
 // ErrWordCount is returned for a length this tool does not offer.
 var ErrWordCount = errors.New("a child mnemonic has 12, 18 or 24 words")
 
+// CheckWords refuses a length this tool does not offer, so that the
+// command can turn it down before it looks for the mnemonic.
+func CheckWords(words uint32) error {
+	switch words {
+	case twelve, eighteen, twentyFour:
+		return nil
+	default:
+		return fmt.Errorf("%w, not %d", ErrWordCount, words)
+	}
+}
+
 // Derive returns the English child mnemonic of that many words at that
 // key index, taken from the master key with BIP-85's own mnemonic
 // application, number 39. The words come straight from the BIP-85
@@ -38,13 +49,12 @@ func Derive(
 	master *hdkeychain.ExtendedKey,
 	words, index uint32,
 ) (string, error) {
-	switch words {
-	case twelve, eighteen, twentyFour:
-	default:
-		return "", fmt.Errorf("%w, not %d", ErrWordCount, words)
+	err := CheckWords(words)
+	if err != nil {
+		return "", err
 	}
 
-	err := derive.CheckIndex(index)
+	err = derive.CheckIndex(index)
 	if err != nil {
 		return "", err
 	}