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/pull_create_test.go

@@ -309,11 +309,9 @@ func TestCreatePullRequestFromNestedOrgForks(t *testing.T) {
 		forkIntoOrg(midForkOrg, leafForkOrg)
 
 		req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", leafForkOrg, repoName, "patch-from-org3.txt"), &api.CreateFileOptions{
-			FileOptions: api.FileOptions{
-				BranchName:    "main",
-				NewBranchName: patchBranch,
-				Message:       "create patch from org3",
-			},
+			BranchName:    "main",
+			NewBranchName: patchBranch,
+			Message:       "create patch from org3",
 			ContentBase64: base64.StdEncoding.EncodeToString([]byte("patch content")),
 		}).AddTokenAuth(token)
 		MakeRequest(t, req, http.StatusCreated)