Add GitHub Actions CI/CD integration guide

[pikespeak]

Goal

Create a guide showing how to use OtterSight CLI in GitHub Actions workflows.

Context

Many users will want to scan their projects on every PR or push. A reusable workflow example makes this easy.

Expected content

A new file at docs/github-actions.md (or a section in the main README) with:

1. A minimal workflow YAML that runs npx @ottersight/cli scan .
2. How to use the Docker image in CI
3. How to fail the build on CRITICAL/HIGH vulnerabilities (exit code behavior)
4. How to upload SARIF results to GitHub Code Scanning (once SARIF format is available)

Getting started

1. Read CONTRIBUTING.md for setup instructions
2. Test the workflow locally with act or in a fork
3. Write clear, copy-pasteable YAML examples

Happy to answer questions in the comments!