fix: a refused share key says what failed, not how to make one

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/share/rules.go

@@ -53,7 +53,7 @@ func (r Rules) Check(key []byte) error {
 	if len(has) == 0 {
 		return nil
 	}
-	return fmt.Errorf("its value has %s; share.keyRules needs %s. Use a random one, like openssl rand -base64 32 makes",
+	return fmt.Errorf("its value has %s; share.keyRules needs %s",
 		strings.Join(has, " and "), strings.Join(needs, " and "))
 }