Add UAT workflow for CI (zxc-uat-test.yaml)

[brunodam]

Create a CI workflow (zxc-uat-test.yaml) that runs user acceptance tests in a QEMU VM on the kvm-runner, mirroring the existing zxc-integration-test.yaml pattern but calling task uat:all instead of task test:integration:verbose.

Background

• UAT tasks (uat:setup, uat:core, uat:teleport, uat:alloy, uat:teardown, uat:all) are designed to run inside any Linux VM
• The existing zxc-integration-test.yaml already handles QEMU VM lifecycle, code sync, proxy tunnels, and cleanup
• The new workflow reuses the same infrastructure but swaps the final SSH command

Scope

• New file: .github/workflows/zxc-uat-test.yaml
• Mirrors zxc-integration-test.yaml steps: VM creation, cloud-init, SSH wait, Go/Task bootstrap, CA cert install, code sync
• Final step SSHs in and runs task uat:all (or individual uat:core, uat:teleport, etc. via input parameter)
• May need longer timeout (~60-90 min) since UAT includes full install/upgrade/uninstall cycles
• Caller workflow in flow-test-integration.yaml or a new flow-test-uat.yaml

Acceptance Criteria

• zxc-uat-test.yaml workflow runs task uat:all successfully in CI
• Supports running individual UAT scenarios via test-name input
• Proxy tunnels work for downloads during install/upgrade steps
• Cleanup step tears down VM regardless of test result