Use wrapper script for label operations in issue triage (#968)

* Use wrapper script for label operations in issue triage

Updates /label-issue command and examples to use a dedicated
edit-issue-labels.sh script for label operations instead of raw
gh issue edit. The script validates labels against the repo's existing
labels before applying them. Also tightens gh search permission to
gh search issues.

* Show multiple --add-label flags in label-issue example

.github/workflows/issue-triage.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Run Claude Code for Issue Triage
         uses: anthropics/claude-code-action@main
         with:
-          prompt: "/label-issue REPO: ${{ github.repository }} ISSUE_NUMBER${{ github.event.issue.number }}"
+          prompt: "/label-issue REPO: ${{ github.repository }} ISSUE_NUMBER: ${{ github.event.issue.number }}"
           anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
           allowed_non_write_users: "*" # Required for issue triage workflow, if users without repo write access create issues
           github_token: ${{ secrets.GITHUB_TOKEN }}