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/cli/migrate/common.go

@@ -24,15 +24,15 @@ import (
 // If the database file doesn't exist, create decides between asking whether
 // to create it (up) and failing (down, status: nothing to work on).
 func openMigrator(
-	cmd *cobra.Command, opts *cli.Options, create bool, createConfirmed bool,
+	cmd *cobra.Command, opts *cli.Options, create, createConfirmed bool,
 ) (*goose.Provider, *sql.DB, error) {
 	conf, err := opts.RunSetup(cmd, true, true)
 	if err != nil {
-		return nil, nil, fmt.Errorf("no configuration loaded (%w)", err)
+		return nil, nil, fmt.Errorf("load configuration: %w", err)
 	}
 
 	file := conf.Database.File
-	if len(file) == 0 {
+	if file == "" {
 		return nil, nil, errors.New(
 			"no database file specified. Update your configuration",
 		)
@@ -46,8 +46,8 @@ func openMigrator(
 			question := fmt.Sprintf(
 				"Database file '%s' does not exist. Would you like to create it?", file,
 			)
-			if !cli.Confirm(question) {
-				return nil, nil, errors.New("migrate: no database file")
+			if !cli.Confirm(cmd.OutOrStdout(), cmd.InOrStdin(), question) {
+				return nil, nil, errors.New("no database file")
 			}
 		}
 	} else if err != nil {
@@ -75,14 +75,14 @@ func closeDB(db *sql.DB, err *error) {
 // errWriter writes until a write fails, then keeps the error, so a run of
 // prints needs checking only once.
 type errWriter struct {
-	w   io.Writer
+	out io.Writer
 	err error
 }
 
 // printf writes like fmt.Fprintf, unless an earlier write failed.
 func (e *errWriter) printf(format string, args ...any) {
 	if e.err == nil {
-		_, e.err = fmt.Fprintf(e.w, format, args...)
+		_, e.err = fmt.Fprintf(e.out, format, args...)
 	}
 }
 
@@ -91,7 +91,7 @@ func (e *errWriter) printf(format string, args ...any) {
 //	applied   00003_create_release_tags_table.sql  (2ms)
 func printResults(out io.Writer, results []*goose.MigrationResult) error {
 	output := &errWriter{
-		w: out,
+		out: out,
 	}
 	for _, result := range results {
 		verb := "applied"
@@ -136,7 +136,9 @@ func failure(out io.Writer, err error) error {
 }
 
 // printVersion reports where the database ended up.
-func printVersion(ctx context.Context, out io.Writer, migrator *goose.Provider) error {
+func printVersion(
+	ctx context.Context, out io.Writer, migrator *goose.Provider,
+) error {
 	dbVersion, err := migrator.GetDBVersion(ctx)
 	if err != nil {
 		return err