Document GitHub Action usage for downstream repositories

[erkininfinity]

Goal

Expand the GitHub Action documentation so users can copy a workflow into their own repositories.

Suggested scope

• Show a minimal workflow using npx agentready-kit check.
• Explain when to use npm install versus package-manager-specific commands.
• Mention pull request and push triggers.

Why this helps

CI integration is one of the clearest ways to keep agent instructions healthy over time.