Fix golangci-lint errcheck warnings; add make fixstyle

errcheck flagged six unchecked Close() calls. The production write
path in indexgen.ProcessDir now closes the index file explicitly
after Execute and surfaces the close error so a deferred flush
failure isn't lost; tests use _ = f.Close() since they don't care.

Also adds a 'make fixstyle' target that runs gofmt -w over the tree,
matching the convention used elsewhere.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/indexgen/indexgen.go

@@ -245,12 +245,13 @@ func ProcessDir(topDir string, opts *Options) error {
 		if err != nil {
 			return fmt.Errorf("cannot create file %s: %w", indexPath, err)
 		}
-		defer file.Close()
-
-		err = GetHTMLTemplate().Execute(file, templateData)
-		if err != nil {
+		if err := GetHTMLTemplate().Execute(file, templateData); err != nil {
+			_ = file.Close()
 			return fmt.Errorf("failed to execute template: %w", err)
 		}
+		if err := file.Close(); err != nil {
+			return fmt.Errorf("failed to close file %s: %w", indexPath, err)
+		}
 	}
 
 	if opts.Recursive {