ci: add Devin on-label automation workflow

[devin-ai-integration]

Summary

Adds a GitHub Actions workflow that automatically creates a Devin session when the devin label is added to any issue in this repo.

How it works

1. Add a devin label to any GitHub issue
2. The workflow calls the Devin API to create a new session
3. A comment is posted on the issue with a link to the Devin session

Prerequisites

• A DEVIN_API_KEY organization secret must be configured in GitHub (Settings → Secrets → Actions → Organization secrets)