chore: update golangci-lint to v2.13.0

Migrate the deprecated gofumpt `extra-rules` to `extra.group-params`, as
the alias now also enables the new `clothe-returns` and `balance-calls`
rules. Disable SA4023, which hangs on go 1.27, see
https://github.com/golangci/golangci-lint/issues/6732.

Apply the resulting modernize fixes, mostly flattening embedded struct
literals and switching errors.As to errors.AsType.

Assisted-by: Claude:Opus 5

tests/integration/api_branch_test.go

@@ -300,11 +300,9 @@ func TestAPIUpdateBranchReference(t *testing.T) {
 		content := base64.StdEncoding.EncodeToString([]byte("branch update test"))
 		var newCommit string
 		doAPICreateFile(ctx, "docs/update.txt", &api.CreateFileOptions{
-			FileOptions: api.FileOptions{
-				BranchName:    defaultBranch,
-				NewBranchName: defaultBranch,
-				Message:       "add docs/update.txt",
-			},
+			BranchName:    defaultBranch,
+			NewBranchName: defaultBranch,
+			Message:       "add docs/update.txt",
 			ContentBase64: content,
 		}, func(t *testing.T, resp api.FileResponse) {
 			newCommit = resp.Commit.SHA