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. Encrypt now takes the parsed recipients. The README says which checks come first.

Model: opus-5-5

internal/cli/options/options.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 
 	"github.com/spf13/cobra"
+	"sneak.berlin/go/keyfunc/internal/derive"
 	"sneak.berlin/go/keyfunc/internal/mnemonic"
 )
 
@@ -23,13 +24,20 @@ func Add(cmd *cobra.Command) {
 	)
 }
 
-// Index returns the key index the user asked for.
+// Index returns the key index the user asked for, and refuses one above
+// derive.MaxIndex. Every command calls it before it looks for the
+// mnemonic.
 func Index(cmd *cobra.Command) (uint32, error) {
 	index, err := cmd.Flags().GetUint32("index")
 	if err != nil {
 		return 0, fmt.Errorf("reading the index: %w", err)
 	}
 
+	err = derive.CheckIndex(index)
+	if err != nil {
+		return 0, err
+	}
+
 	return index, nil
 }