Land the multi-repo e2e harness (test/e2e)

A configurable multi-repo end-to-end harness lives on branch `pr/e2e-harness` (`test/e2e/`). It does a real Cognito SRP login + REST API drive of the full inception → construction → review → PR flow across multiple repos, with a Playwright observer that screenshots the real UI at every step.

Scenarios: `single-repo`, `multi-changed`, `no-change`, `review-cycle`. Artifacts: per-run `report.html` + screenshots + `events.jsonl` + `summary.json`.

It was used to validate the multi-repo rebase against staging (and surfaced #239). Filed as follow-up rather than folding into the multi-repo PR stack to keep those PRs focused.

Branch: `pr/e2e-harness` (stacked on the multi-repo stack). Decide whether to land as its own PR or fold into the test suite.

Inspect branch `pr/e2e-harness` and the existing files under `test/e2e/` first, then review how the four named scenarios and their generated artifacts are configured. Done means deciding whether the harness lands as its own PR or joins the test suite, with the scenarios and reports remaining runnable against the intended staging flow.

Written by the indexing model from the issue text.