config init now leaves out the test sources unless given --all

A section tagged init:"hidden" isn't written to the file config init
generates; fts_mock and sleeper are. config init --all writes them too.
They still load, and the config commands read and set them as before.

internal/config/section.go

@@ -50,6 +50,10 @@ const (
 	keyTag = "cfg"
 	// defaultTag holds a field's default value.
 	defaultTag = "default"
+	// initTag set to initHidden on a section field leaves the section out of
+	// the file `config init` writes (see InitDefaults), as for test sources.
+	initTag    = "init"
+	initHidden = "hidden"
 )
 
 // The unknown key error and the field types section.go handles specially.
@@ -109,6 +113,8 @@ type field struct {
 	section      bool
 	defaultValue string
 	hasDefault   bool
+	// hiddenFromInit is set by initTag (see InitDefaults).
+	hiddenFromInit bool
 }
 
 // isSectionList reports whether listType is a list of sections, []S for a
@@ -146,8 +152,9 @@ func fieldsOf(structType reflect.Type) []field {
 			fieldType: structField.Type,
 			section: structField.Type.Kind() == reflect.Struct &&
 				structField.Type != timeType,
-			defaultValue: defaultValue,
-			hasDefault:   hasDefault,
+			defaultValue:   defaultValue,
+			hasDefault:     hasDefault,
+			hiddenFromInit: structField.Tag.Get(initTag) == initHidden,
 		})
 	}
 
@@ -244,6 +251,25 @@ func flattenInto(flat map[string]any, prefix string, section reflect.Value) {
 	}
 }
 
+// hiddenFromInitKeys returns the dotted keys of the sections under section
+// (prefixed with prefix, if non-empty) whose field has initTag set to
+// initHidden, descending into nested sections.
+func hiddenFromInitKeys(prefix string, sectionType reflect.Type) []string {
+	var hidden []string
+	for _, configField := range fieldsOf(sectionType) {
+		if !configField.section {
+			continue
+		}
+		key := joinKey(prefix, configField.key)
+		if configField.hiddenFromInit {
+			hidden = append(hidden, key)
+			continue
+		}
+		hidden = append(hidden, hiddenFromInitKeys(key, configField.fieldType)...)
+	}
+	return hidden
+}
+
 // exportValue returns a field's value as Map, Flatten and ValueOf give it
 // out: durations and byte sizes become strings such as "1m30s" and "32KiB",
 // so values agree with kindOf and can be passed back to SetValue; []string is