Add OAI Coding Agent GitHub Workflow

[mattmorgis]

This PR adds a GitHub workflow that automatically responds to issues when:

- An issue is opened with `@oai` in the body
- An issue is labeled with `oai`
- A comment on an issue contains `@oai`

The workflow will:
1. Create a new branch for the issue
2. Run the OAI coding agent to address the issue
3. Create a pull request with the solution