STAMP: Add deploy test to GitHub Actions CI/CD

[Ultimate-Storm]

Context

The ODELIA deploy test has a GitHub Actions workflow (odelia-deploy-test.yml) that runs automatically. The STAMP deploy test currently only runs manually via:

```bash
./scripts/deploy/run_stamp_deploy_test.sh --conf deploy_sites_2node_stamp_test.conf
```

It was validated once manually (2-round VIT swarm training, 2 nodes, PASSED in 127s) but is not part of the CI/CD pipeline.

Proposed Solution

1. Create .github/workflows/stamp-deploy-test.yml (modeled on odelia-deploy-test.yml)
2. Trigger on:
   • Push to main when STAMP-related files change
   • Manual dispatch (workflow_dispatch)
   • Nightly/weekly schedule
3. Include Docker image build, synthetic data distribution, and 2-node deploy test
4. Add status badge to README.md

Prerequisites

• The deploy test machines (DL0, DL2) and Cosmos server must be accessible from the GitHub Actions runner (currently they are on a Tailscale VPN)
• Consider whether self-hosted runners are needed (same as ODELIA deploy test)

Related Files

• scripts/deploy/run_stamp_deploy_test.sh — STAMP deploy test script
• deploy_sites_2node_stamp_test.conf — site configuration
• .github/workflows/odelia-deploy-test.yml — ODELIA CI/CD workflow (template)