ci(labels): create unforced labels only when they are missing

gh label create exits non-zero for a label that already exists, and the
step runs with bash -e, so the first existing unforced label stopped the
job. Skip labels that already exist, keeping hand-set colors.

Fixes #2320

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/labels.yml

@@ -19,7 +19,8 @@ jobs:
       GH_REPO: ${{ github.repository }}
     steps:
       - run: |
-          label() { gh label create "$1" --color "$2" --description "$3"; }
+          existing=$(gh label list --limit 1000 --json name --jq '.[].name')
+          label() { grep -qixF -- "$1" <<< "$existing" || gh label create "$1" --color "$2" --description "$3"; }
           force_label() { gh label create "$1" --color "$2" --description "$3" --force; }
 
           force_label 'needs rebase' b60205 'Conflicts with main, rebase needed'