🚀 Automate GCP deploy with GitHub Actions + OIDC

[gepser]

✨ Summary

This PR implements automated deployment to Google Cloud Functions Gen2 using GitHub Actions and Workload Identity Federation (OIDC), so deploys no longer depend on manual local commands.

Closes #15

✅ Changes

• Added deploy workflow:
  • .github/workflows/deploy.yml
  • Triggers on push to master and workflow_dispatch
  • Uses google-github-actions/auth@v3
  • Uses google-github-actions/deploy-cloud-functions@v4
  • Includes post-deploy smoke test step
• Improved smoke script portability:
  • scripts/smoke.sh
  • Supports both base-domain mode and direct function URL mode via PROGRESS_PATH
• Expanded README setup docs:
  • README.md
  • Added OIDC/WIF one-time setup steps
  • Added required GitHub repository variables
  • Added automated deploy workflow usage

🧪 Validation

• mise exec -- make check
• Local smoke test (root base URL mode):
  • BASE_URL=http://localhost:8080 bash ./scripts/smoke.sh
• Local smoke test (direct function URL mode):
  • BASE_URL=http://localhost:8080/progress PROGRESS_PATH="" bash ./scripts/smoke.sh

🔐 Security posture

• No long-lived JSON credentials.
• Deploy uses GitHub OIDC + Workload Identity Federation only.
• Scoped to repository-level identity binding (documented in README).