e2e: revisit the three remaining config-absence skips once studio-e2e-harness#2 lands

Follow-up from the review of #1572 (David: *"Three config-absence skips remain and are now documented rather than implicit — that's the right resting place until harness#2 lands."*).

#1572 converted every **ambiguous** skip to a failure. What deliberately remains are three **absence-of-configuration** skips, which are legitimate today because nothing else backstops them:

- `org-users.authed.spec.ts` — `test.skip(!hasCreds, …)`
- `signup-verification.anon.spec.ts` — `test.skip(!mailConfigured, …)`
- `auth.setup.ts` — writes an empty storage state and skips when creds are absent

The risk they carry: **rotate the test account password and the authed lane goes quiet and green.** The intended backstop is the harness failing any run with `skipped > 0`, which lives in
[studio-e2e-harness#2](https://github.com/HarperFast/studio-e2e-harness/pull/2) and is **not on that repo's `main`** yet.

When harness#2 merges:
1. Drop the "not on that repo's `main` yet" caveat from the `## Skips are deliberate, and narrow` section of `e2e/README.md`.
2. Decide whether the three skips should additionally become hard failures under an explicit `E2E_REQUIRE_COVERAGE=1` that the lanes set — belt-and-braces, so the guarantee does not depend solely on a runner-side check in another repo.

Wait for studio-e2e-harness#2 to land, then read the three skip sites: org-users.authed.spec.ts, signup-verification.anon.spec.ts, and auth.setup.ts. Update the caveat in e2e/README.md, decide whether E2E_REQUIRE_COVERAGE=1 is also needed, and verify that absent configuration cannot leave the relevant lanes green with skipped tests.

Written by the indexing model from the issue text.