Add GitHub Action for cora in CI/CD

[ajianaz]

Description

Create a reusable GitHub Action that wraps cora-cli for easy CI/CD integration.

Acceptance Criteria

• Publish as ajianaz/cora-action (separate repo)
• Action accepts inputs: model, severity, output_format, api_key
• Supports SARIF upload to GitHub Code Scanning via composite action
• Document usage in cora-cli README with examples
• Support PR comment posting with review results

Example Usage

- uses: ajianaz/cora-action@v1
  with:
    api_key: ${{ secrets.OPENAI_API_KEY }}
    severity: error
    upload_sarif: true

[ajianaz]

Implemented in PR #24, merged to develop.