Add OAI Coding Agent GitHub Workflow

[mattmorgis]

This PR adds a GitHub workflow that automatically responds to:

- An issue is labeled with `oai`
- A comment on an issue or PR that contains `@oai`
- A "changes requested" review on a PR authored by the agent

The workflow will:
1. Set up the required environment for the agent (Node.js, uv, oai-coding-agent)
2. For issues: Create a new branch and PR with the solution
3. For PR comments/reviews: Update the existing PR branch with requested changes