Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/config/config_test.go

@@ -10,7 +10,10 @@ import (
 
 // Kinds that LoadFromViper knows how to read.
 var viperKinds = map[reflect.Kind]bool{
-	reflect.String: true, reflect.Int: true, reflect.Bool: true, reflect.Slice: true,
+	reflect.String: true,
+	reflect.Int:    true,
+	reflect.Bool:   true,
+	reflect.Slice:  true,
 }
 
 // Every key round-trips through TypeOf/ValueOf/SetValue and has a type that
@@ -25,7 +28,9 @@ func TestKeysRoundTrip(t *testing.T) {
 		if err != nil {
 			t.Errorf("TypeOf(%q): %v", key, err)
 		} else if !viperKinds[kind] {
-			t.Errorf("TypeOf(%q) = %v, which LoadFromViper does not handle", key, kind)
+			t.Errorf(
+				"TypeOf(%q) = %v, which LoadFromViper does not handle", key, kind,
+			)
 		}
 		got, err := conf.ValueOf(key)
 		if err != nil || !reflect.DeepEqual(got, want) {
@@ -53,6 +58,7 @@ func TestSections(t *testing.T) {
 	if !reflect.PointerTo(reflect.TypeOf(conf)).Implements(sectionType) {
 		t.Error("*JobSearchConfig does not implement Section")
 	}
+
 	all := conf.Flatten()
 	uses := map[reflect.Type]int{}
 	countSections(uses, reflect.TypeOf(conf))
@@ -64,8 +70,8 @@ func TestSections(t *testing.T) {
 func countSections(uses map[reflect.Type]int, section reflect.Type) {
 	for _, f := range fieldsOf(section) {
 		if f.section {
-			uses[f.typ]++
-			countSections(uses, f.typ)
+			uses[f.fieldType]++
+			countSections(uses, f.fieldType)
 		}
 	}
 }
@@ -78,14 +84,16 @@ func checkSections(
 	prefix string,
 	section reflect.Value,
 ) {
+	t.Helper()
 	for _, f := range fieldsOf(section.Type()) {
 		key := joinKey(prefix, f.key)
-		if isSectionList(f.typ) {
+		if isSectionList(f.fieldType) {
 			checkSectionList(t, all, key, section.FieldByIndex(f.index))
 		}
 		if !f.section {
 			continue
 		}
+
 		fv := section.FieldByIndex(f.index)
 		name := fv.Type().Name()
 		if !fv.Addr().Type().Implements(sectionType) {
@@ -93,18 +101,16 @@ func checkSections(
 			continue
 		}
 		checkEmbedded(t, fv.Type())
+
 		keys := fv.Addr().Interface().(Section).Flatten()
 		keyPrefix := prefix
-		if uses[f.typ] > 1 {
+		if uses[f.fieldType] > 1 {
 			keyPrefix = key
 		}
 		for k := range keys {
 			if _, ok := all[joinKey(keyPrefix, k)]; !ok {
 				t.Errorf(
-					"%s.Flatten() key %q does not match its cfg tag %q",
-					name,
-					k,
-					key,
+					"%s.Flatten() key %q does not match its cfg tag %q", name, k, key,
 				)
 			}
 		}
@@ -126,29 +132,36 @@ func checkSections(
 
 // checkSectionList checks the items of the list of sections at key implement
 // Section and that their methods agree with the list's value in all.
-func checkSectionList(t *testing.T, all map[string]any, key string, list reflect.Value) {
+func checkSectionList(
+	t *testing.T, all map[string]any, key string, list reflect.Value,
+) {
 	t.Helper()
 	name := list.Type().Elem().Name()
 	if !reflect.PointerTo(list.Type().Elem()).Implements(sectionType) {
 		t.Errorf("*%s (items of %s) does not implement Section", name, key)
 		return
 	}
+
 	items, _ := all[key].([]map[string]any)
 	if len(items) != list.Len() {
 		t.Errorf("%s has %d items; Flatten gives %d", key, list.Len(), len(items))
 		return
 	}
+
 	for i := range list.Len() {
 		item, itemName := list.Index(i), fmt.Sprintf("%s[%d]", key, i)
 		want := flattenMap("", items[i])
+
 		// Items are not prefixed, and their kinds are their fields' kinds.
-		if got := item.Addr().Interface().(Section).Flatten(); !reflect.DeepEqual(got, want) {
+		got := item.Addr().Interface().(Section).Flatten()
+		if !reflect.DeepEqual(got, want) {
 			t.Errorf("%s.Flatten() =\n%v\nwant\n%v", itemName, got, want)
 		}
+
 		kinds := map[string]reflect.Kind{}
 		for k := range want {
 			_, f, _ := lookup(item, k)
-			kinds[k] = kindOf(f.typ)
+			kinds[k] = kindOf(f.fieldType)
 		}
 		checkSectionMethods(t, itemName, item, want, kinds)
 	}
@@ -164,10 +177,12 @@ func checkSectionMethods(
 	want map[string]any,
 	kinds map[string]reflect.Kind,
 ) {
+	t.Helper()
 	s := section.Addr().Interface().(Section)
 	if got := flattenMap("", s.Map()); !reflect.DeepEqual(got, want) {
 		t.Errorf("%s.Map() =\n%v\nwant\n%v", name, got, want)
 	}
+
 	before := section.Interface()
 	for k, v := range want {
 		if kind, err := s.TypeOf(k); err != nil || kind != kinds[k] {
@@ -180,6 +195,7 @@ func checkSectionMethods(
 			t.Errorf("%s.SetValue(%q, %#v): %v", name, k, v, err)
 		}
 	}
+
 	if !reflect.DeepEqual(section.Interface(), before) {
 		t.Errorf("%s changed by setting its keys to their own values", name)
 	}