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 {

internal/indexgen/indexgen_test.go

@@ -271,7 +271,7 @@ func TestReadDirEntries(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to write to file: %v", err)
 		}
-		f.Close()
+		_ = f.Close()
 	}
 
 	// Create a subdirectory
@@ -328,7 +328,7 @@ func TestReadDirEntriesWithHidden(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to create test file %s: %v", file, err)
 		}
-		f.Close()
+		_ = f.Close()
 	}
 
 	opts := &Options{
@@ -365,7 +365,7 @@ func TestReadDirEntriesWithRegexExclusion(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to create test file %s: %v", file, err)
 		}
-		f.Close()
+		_ = f.Close()
 	}
 
 	regex := regexp.MustCompile("(build|node_modules)")

internal/indexgen/integration_test.go

@@ -22,7 +22,7 @@ func TestProcessDirBasic(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to write to file: %v", err)
 		}
-		f.Close()
+		_ = f.Close()
 	}
 
 	opts := &Options{
@@ -85,7 +85,7 @@ func TestProcessDirRecursive(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create root file: %v", err)
 	}
-	f.Close()
+	_ = f.Close()
 
 	// Create subdirectory with files
 	subDir := filepath.Join(tempDir, "subdir")
@@ -98,7 +98,7 @@ func TestProcessDirRecursive(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create sub file: %v", err)
 	}
-	subFile.Close()
+	_ = subFile.Close()
 
 	// Create nested subdirectory
 	nestedDir := filepath.Join(subDir, "nested")
@@ -111,7 +111,7 @@ func TestProcessDirRecursive(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create nested file: %v", err)
 	}
-	nestedFile.Close()
+	_ = nestedFile.Close()
 
 	opts := &Options{
 		TopDir:        tempDir,
@@ -174,7 +174,7 @@ func TestProcessDirWithExcludeRegex(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to create test file %s: %v", file, err)
 		}
-		f.Close()
+		_ = f.Close()
 	}
 
 	regex := regexp.MustCompile("(tmp|node_modules)")
@@ -259,7 +259,7 @@ func TestProcessDirVerbose(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create test file: %v", err)
 	}
-	f.Close()
+	_ = f.Close()
 
 	opts := &Options{
 		TopDir:        tempDir,
@@ -308,7 +308,7 @@ func TestProcessDirWithSymlinks(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to write to file: %v", err)
 	}
-	f.Close()
+	_ = f.Close()
 
 	// Create a symlink to the file (skip on Windows)
 	symlinkFile := filepath.Join(tempDir, "symlink.txt")