Fix prPreview.yml: drop gh run cancel/watch, use exit 1 for missing-secret guard

[Copilot]

gh run cancel requires actions: write, but the workflow only declares contents: write, causing a 403 when secrets are absent. The file was also fully commented out.

Changes

• Restore workflow: uncomment the entire prPreview.yml definition
• Fix permission issue: replace gh run cancel ${{ github.run_id }} && gh run watch ${{ github.run_id }} with exit 1 in both secret-check steps — fails the job normally with no additional permissions required

# Before
- if: ${{ env.INDEXER_URL == '' }}
  run: |
    echo "Missing INDEXER_URL"
    gh run cancel ${{ github.run_id }}
    gh run watch ${{ github.run_id }}

# After
- if: ${{ env.INDEXER_URL == '' }}
  run: |
    echo "Missing INDEXER_URL"
    exit 1

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