feat(orchestrator): migrate the buildsignal poll loop to the hold primitive

[behinddwalls]

Summary

Why?

The orchestrator's buildsignal stage carried the same ack-and-republish poll loop the stovepipe stages just migrated off (previous commit): each non-terminal poll acked the delivery and PublishAfter'd the build id back to its own topic. Notably its republish reused the build id as the message id — the exact dedup collision that stalled stovepipe's loop (#465) was latent here — and a transient publish failure was wrapped retryable as the loop's only liveness, the awkward classification #469 removed on the stovepipe side.

What?

A non-terminal status now records a hold for the per-status poll delay and returns success; the framework postpones the delivery, which redelivers without counting toward the retry limit. publishBuild is deleted (the re-poll was its only caller); the speculate publish and halted-batch short-circuit are unchanged, and the DLQ reconciler is unaffected. The Process retryability comment is rewritten: the loop's continuation is framework-owned, so no publish needs a retryable wrap. build-runner.md's "Polling primitive" section is updated — hold supersedes the PublishAfter design it argued for, keeping the same retry_count semantics with no publisher, no minted ids, and no per-tick rows.

Test Plan

✅ bazel test //submitqueue/... — NonTerminal cases assert Hold(per-status delay); Terminal/StatusError/UpdateStatusError/Halted cases fail on any Hold; the RepublishError test is deleted (a hold cannot fail). ✅ make fmt.

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