feat(messagequeue)!: remove PublishAfter and the visible_after column

[behinddwalls]

Summary

Why?

PublishAfter existed for one pattern: a consumer acking its own delivery and republishing a delayed copy to keep a poll or gate-wait loop alive. The two previous commits migrated all three such loops (stovepipe process/buildsignal, orchestrator buildsignal) to the hold primitive, leaving PublishAfter with zero callers. Removing it shrinks the publisher contract to a single verb and deletes the delayed-visibility machinery that existed only to serve it.

What?

Publisher loses PublishAfter; the mysql impl deletes its method, folds InsertDelayed back into Insert, drops FetchByOffset's nowMs parameter and its visible_after filter, removes the column from the queue_messages schema and the MoveToDLQ column list, and regenerates mocks. Tests covering delayed publish/fetch are deleted; the misnamed TestPublisher_PublishAfterClose (which tests Publish on a closed publisher) is renamed. READMEs drop the PublishAfter guidance — a consumer defers its own delivery with Hold/Postpone, and no caller needed cross-topic delayed publishing.

Deployment note: the schema files here are applied by dev/test stacks; a live database needs ALTER TABLE queue_messages DROP COLUMN visible_after (old rows with a pending visible_after would become immediately visible on rollout, which is the correct degradation — those rows were minted by the pre-hold republish pattern and their consumers supersede/no-op them).

Test Plan

✅ make test (83 targets). ✅ bazel test //test/integration/extension/messagequeue/... — full queue behavior including idempotent publish, DLQ, postpone barrier. ✅ make fmt, make mocks.

Stack

1. docs(rfc): consumer hold — postpone a delivery as a partition barrier #486
2. feat(platform): hold/postpone — a fourth delivery outcome for backing off #487
3. feat(stovepipe): migrate process and buildsignal waits to the hold primitive #488
4. feat(orchestrator): migrate the buildsignal poll loop to the hold primitive #489
5. @ feat(messagequeue)!: remove PublishAfter and the visible_after column #490
6. feat(messagequeue)!: drop the requeue delay parameter from Nack #491
7. feat(consumergate)!: rebuild the gate on the postpone primitive #492