groups: delete the dead plural ui/groups scries

[arthyn]

Summary

Removes the plural ui/groups %groups scry arm, which has no consumers, rather than carrying it forward to v3. Follow-up to #6209 (TLON-6274), stacked on it — review that one first.

Comes out of @mikolajpp's review, threads on the plural ui/groups ("we don't expose v11 here") and on light/groups ("Is this endpoint in use by anything? If not, we should mark it for removal later. Otherwise it needs updating too").

light/groups turned out to be in use, so it stays. The first pass deleted it on a bad audit and broke CI — see below.

Changes

• Delete the plural ui/groups peek arm and its three scry declarations — /x/v0|v1|v2/ui/groups
• Repoint urbitRequestTimeout.test.ts at /v3/groups; it used /v2/ui/groups as arbitrary filler against a stubbed fetch and would otherwise name a deleted endpoint

Net 18 lines removed from app/groups.hoon.

Why light/groups stays

The first commit deleted it too, and both the parallel E2E job and the production build smoke test failed in "Setting up ships" with ~zod is not ready / ~ten is not ready until Max attempts reached. Cause: rube's own liveness probe reads it.

apps/tlon-web/rube/index.ts:1205
  http://localhost:${ship.httpPort}/~/scry/groups/groups/light.json

checkGroupsAppHealth 404s → shipsAreReadyForTests never returns true → 30 attempts → rube aborts before any test runs.

Three more live callers depend on the same deprecated alias:

Caller	Path
apps/tlon-mobile/ios/Shared/Networking/PocketChatAPI.swift:17	/~/scry/groups/groups/light
apps/tlon-mobile/android/app/src/main/java/io/tlon/landscape/api/TalkApi.java:137	/~/scry/groups/groups/light
api/openapi.yaml:42	documented as /groups/groups/light.json

The audit missed all four because it grepped the versioned spelling (light/groups); every real caller uses the alias spelling (groups/light). The arm and its four declarations are restored verbatim.

On @mikolajpp's "otherwise it needs updating too": every caller reads it through the unversioned alias, which resolves to %v0 and the %groups mark. Nothing reads %v1/%v2, and nothing needs v11/blob from it — so there is nothing to update, just something not to delete.

How did I test?

The plural ui/groups arm is dead: the only hit was the timeout test above. The two in-desk callers (ted/group/create.hoon, ted/group/create-1.hoon) read the per-flag path /gx/groups/vN/ui/groups/<ship>/<name>, a different arm that stays.

On a live moon: desk compiles and %groups loads, and the surviving per-flag arm still answers — /v3/ui/groups/{flag} and /v3/groups both return correctly. That was the real risk, since the deleted plural arm shared the %ui %groups prefix.

./backend/run-tests.sh green on the first pass: 319 OK, Unit tests passed ✅ and Aqua tests passed ✅. Affected vitest file passes (6/6), Prettier clean. The restore commit only adds back code that compiled at the base, so re-running CI is the check that matters — specifically that E2E and the smoke test get past ship setup.

Risks and impact

• Safe to rollback without consulting PR author? (Yes)
• Affects important code area:
  • Onboarding
  • State / providers
  • Message sync
  • Channel display
  • Notifications
  • Other: removes one unused %groups scry endpoint

A third-party client scrying /x/v0|v1|v2/ui/groups would start getting no-match. These are unversioned-through-v2 legacy reads with no consumer in this repo, and the per-flag equivalents remain.

Note this orphans the %groups-ui, %groups-ui-1, and %groups-ui-2 marks — nothing produces them now. I deliberately left the mark files and their strict-list entries alone, since dropping a strict mark is a one-way door (it can never go strict again without nest-failing legacy ships) and keeping them costs nothing.

Worth a separate look: rube's health probe hanging off a deprecated alias is fragile — it made a legacy-cleanup PR look like an infrastructure outage. Repointing it at a supported endpoint would make the next cleanup safer.

Rollback plan

Revert the PR. No state, migration, or wire-format change.

[linear-code]

TLON-6274