fix(ci): gate the stand run on the label just added, not the whole label set

The job triggers on `pull_request: [labeled]`, but the guard tested
`contains(labels.*.name, 'e2e:stand')` — the label *set*, not the label that fired
the event. So adding ANY label to a PR that already carries `e2e:stand` re-triggered
the run, and `cancel-in-progress` then killed the approved, in-flight stand run and
re-pinged the environment reviewers. Gates on `github.event.label.name` instead;
workflow_dispatch is unchanged, and the job only listens to labeled + dispatch so the
event always carries a label name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/e2e-stand.yml

@@ -30,7 +30,7 @@ jobs:
   e2e-stand:
     if: >
       github.event_name == 'workflow_dispatch' ||
-      contains(github.event.pull_request.labels.*.name, 'e2e:stand')
+      github.event.label.name == 'e2e:stand'
     runs-on: ubuntu-latest
     timeout-minutes: 30
     environment: e2e-stand