stovepipe: keep queue contracts proto-only — retire entity JSON serializers, contracts for upcoming stages

[behinddwalls]

Status

The stovepipe domain is mostly on the proto queue-contract convention (doc/rfc/messagequeue-contract.md): its one live topic, process, uses ProcessRequest from stovepipe/core/messagequeue/ with protojson glue, topic_keys binding, and the reference layout. This issue covers the remaining gaps so the domain ends up — and stays — proto-only.

Work

1. Retire the leftover JSON payload serializers. stovepipe/entity/request.go still exposes ToBytes/FromBytes on Request and RequestID, documented as "for queue message payload" — but no queue uses them anymore (ingest publishes ProcessRequest proto; process consumes it). Remove them (or, if some non-queue use exists, re-document them so they stop advertising themselves as queue serialization). As long as they exist, the next stage's author has an attractive wrong tool one import away.
2. Proto contracts for upcoming stages from day one. process.go carries the TODO "re-publish to build once the build stage lands (RFC process algorithm, step 3)" — the build stage (and any later stovepipe queues) must define their payloads in stovepipe/core/messagequeue/ with topic_keys bindings before the controller lands, extending the existing contract test (round-trip + one message per topic key) rather than reaching for entity JSON.
3. Audit in-flight stovepipe PRs (e.g. the admit-latest-head-through-process work) for any queue payload that bypasses the proto contract package, and route them through it before merge.

Caveats

• Same wire-format note as the submitqueue counterpart: if any transitional JSON payload does exist on a live queue, cutover needs a drained queue or dual-format read — currently believed to be none for stovepipe.
• The message-ID convention (messagequeue: entity-ID message IDs + publish dedup silently drop wake-up events; use intent-scoped message IDs #352, intent-scoped message IDs) is orthogonal but touches the same publish sites; coordinate.

Related

• messagequeue: entity-ID message IDs + publish dedup silently drop wake-up events; use intent-scoped message IDs #352 — intent-scoped message IDs
• submitqueue: port internal queue contracts to proto (submitqueue/core/messagequeue) #356 — submitqueue counterpart (11 of 12 topics still on entity JSON)

[behinddwalls]

Linear: CODEM-306