Refuse config keys that no setting has (closes #419)

config.Parse walks the file as written and fails on the first key that
matches no setting, naming its dotted path and line, so a typo in
exclude or prune_grace_period no longer falls back silently. Keys that a
${...} reference brings in are checked once smartconfig has filled it
in, and named without a line. YAML merge keys and a top-level env
section, which smartconfig reads, still load. config set refuses such a
key before writing. The key-to-field lookup moves from internal/cli to
config.SettingType so both use it.

Model: opus-5-5

internal/cli/config.go

@@ -46,6 +46,7 @@ var (
 	errNeedNumericIndex = errors.New("key is a list; use a numeric index")
 	errIndexOutOfRange  = errors.New("index out of range")
 	errNotMapOrList     = errors.New("key is not a map or list")
+	errNoSuchSetting    = errors.New("no setting has this key")
 	errWouldNotLoad     = errors.New(
 		"vaultik would not load the config after this change, " +
 			"so the file was not changed")
@@ -443,11 +444,10 @@ number, otherwise as a map.
 
 If the config loads, a change after which it would not, such as
 compression_level 99, is refused and the file is left unchanged. A
-config that does not load, such as the one config init writes before
-storage_url is set or one with a ${ENV:...} reference to a variable
-not set here, is checked one setting at a time: only a change after
-which the changed setting cannot be read, such as one giving it the
-wrong type, is refused.
+config that does not load is checked one setting at a time. Examples
+are the file config init writes before storage_url is set, and one
+with a ${ENV:...} reference to a variable not set in this shell. In
+such a config, an unknown key or a value of the wrong type is refused.
 
 Examples:
   vaultik config set storage_url "file:///mnt/backups"
@@ -484,6 +484,11 @@ func writeConfigSet(out *ui.Writer, path, key, value string) error {
 
 	keys := strings.Split(key, ".")
 
+	_, ok := config.SettingType(keys)
+	if !ok {
+		return fmt.Errorf("%w: %s", errNoSuchSetting, key)
+	}
+
 	err = yamlPathSet(root, keys, value)
 	if err != nil {
 		return err
@@ -739,41 +744,11 @@ func yamlPathSet(root *yaml.Node, keys []string, value string) error {
 }
 
 // configKeyIsString reports whether the dotted key names a string in
-// config.Config, following the fields' yaml tags, as s3.access_key_id and
-// snapshots.home.exclude.0 do.
+// config.Config, as s3.access_key_id and snapshots.home.exclude.0 do.
 func configKeyIsString(keys []string) bool {
-	typ := reflect.TypeFor[config.Config]()
+	typ, ok := config.SettingType(keys)
 
-	for _, key := range keys {
-		switch {
-		case typ.Kind() == reflect.Map || typ.Kind() == reflect.Slice:
-			// The key is a snapshot name or a list index.
-			typ = typ.Elem()
-		case typ.Kind() == reflect.Struct:
-			field, ok := yamlField(typ, key)
-			if !ok {
-				return false
-			}
-
-			typ = field.Type
-		default:
-			return false
-		}
-	}
-
-	return typ.Kind() == reflect.String
-}
-
-// yamlField returns the field of struct type typ whose yaml tag names key.
-func yamlField(typ reflect.Type, key string) (reflect.StructField, bool) {
-	for field := range typ.Fields() {
-		name, _, _ := strings.Cut(field.Tag.Get("yaml"), ",")
-		if name == key {
-			return field, true
-		}
-	}
-
-	return reflect.StructField{}, false
+	return ok && typ.Kind() == reflect.String
 }
 
 // yamlSetInMapping resolves (creating if needed) the value node for the