ci: per-issue concurrency group for auto-label workflow

The auto-label-issues concurrency group was ${workflow}-${ref}, but for
issues: events github.ref is the default branch for EVERY issue, so the
group was global. With cancel-in-progress:true, a burst of new issues
(each opened seconds apart) cascaded cancellations — only the last issue
in the burst survived and got needs-triage; earlier ones were cancelled
mid-flight and left unlabeled. Observed twice today: #1911/#1912 (nabki
batch) and the #1900/#1904 epic bursts.

Make the group per-issue (github.event.issue.number) so runs no longer
collide across issues. cancel-in-progress is now per-issue (harmless).

.github/workflows/auto-label-issues.yml

@@ -5,7 +5,7 @@ on:
     types: [opened]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.issue.number }}
   cancel-in-progress: true
 
 jobs: