Create reusable GitHub Action for nightly index updates

[jonyoder]

Publish a reusable GitHub Action (posit-dev/code-index-action) so any repo can add nightly index updates:

- uses: posit-dev/code-index-action@v1
  with:
    config: .code-index.json

The action should:

• Install the code-index binary (from GitHub Releases)
• Pull existing data from storage (S3 or HTTP URL)
• Run the full pipeline (parse → generate → build → embed)
• Upload updated data to storage
• Optionally create a PR with updated checked-in files