Replace the stub placeholder with a comprehensive integration test
script that verifyies Docker daemon, compose stack, and service
endpoint health. All configuration via environment variables with
sensible defaults.
Changes:
- tests/run-integration.sh: 5-phase test suite with color output,
retry logic, env-var configuration, and CI-friendly exit codes
- .gitea/workflows/build-nixos.yml: update CI step to document
pr-test-vm usage with the new test script
See also: pr-test-vm helper in modules/nixos/services/staging-vm.nix
