Quality: end-to-end test suite for v5

v5 needs an E2E suite that exercises real workload install + project
init + run flows across the supported stacks. Scope, build, and wire
into CI.

## Definition of Done

- [ ] E2E framework chosen + scaffolded
- [ ] Core scenarios covered per stack (init, install workload, start,
publish)
- [ ] CI job runs E2E on PR + main
- [ ] Flake budget + retry policy defined

No files, existing tests, or CI entry points are named. Start by mapping the current project-init, workload-install, run, and publish flows and the supported stacks, then review how CI is configured. Done means an E2E framework is scaffolded, core scenarios run for each stack, CI executes them on PRs and main, and flake and retry policies are defined.

Written by the indexing model from the issue text.