chore: set up auto-labeling for new issues

[andreahlert]

Part of #690

Create a GitHub Actions workflow that automatically applies status/needs-triage to new issues. This ensures nothing falls through the cracks and makes the triage queue visible.

Proposed workflow

.github/workflows/auto-label.yml:

name: Auto-label new issues
on:
  issues:
    types: [opened]

permissions:
  issues: write

jobs:
  label:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@v7
        with:
          script: |
            await github.rest.issues.addLabels({
              owner: context.repo.owner,
              repo: context.repo.repo,
              issue_number: context.issue.number,
              labels: ['status/needs-triage']
            })

Future automation (optional)

• Auto-apply area/* labels based on file paths changed in PRs (similar to Airflow's boring-cyborg)
• Auto-apply pr/needs-rebase when a PR has merge conflicts
• Weekly stale check to auto-apply lifecycle/stale after 90 days of inactivity