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_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)")