docs(rfc): stovepipe post-merge trunk-validation workflow

## Summary

### Why?

SubmitQueue can no longer prove every change green before merge at the monorepo's current throughput, so it now merges directly to a single `main` that may be temporarily broken. Stovepipe is the post-merge service that validates `main`, records per-commit health, and drives recovery. There was no design doc describing its pipeline; this RFC fills that gap, mirroring `doc/rfc/submitqueue/workflow.md` so the two services read as siblings.

### What?

Adds `doc/rfc/stovepipe/workflow.md` describing the end-to-end pipeline: ingest trunk push events (external webhooks plus a fallback reconciliation poller, deduped on commit SHA) → start → validate → batch commits since the last known green → speculate / build / buildsignal → on green record `succeeded`; on failure bisect to the offending commit → invoke a pluggable remediation extension whose external backend lands a fix or revert via SQ.

Documents the three commit states (`unknown` / `succeeded` / `failed`), the SHA-as-identity / batch-as-validation-unit tracking model (bisect owns termination), and two gateway-owned sinks the orchestrator publishes to and the gateway consumes: `status` (the commit-status store callers query) and `log` (an append-only event log, the analogue of SQ's request log). Also covers fail-closed DLQ reconciliation, ownership by service, and the status/log ownership invariant. Links the new doc from `doc/rfc/index.md` under a new `## Stovepipe` section.

Author: behinddwalls

doc/rfc/index.md

@@ -10,3 +10,7 @@ Design documents and technical proposals, grouped by scope. Shared/cross-cutting
 
 - [Orchestrator Workflow](submitqueue/workflow.md) - Queue-driven controller pipeline from gateway entry through batching, scoring, build, merge, and conclude
 - [Build Runner](submitqueue/build-runner.md) - Vendor-agnostic BuildRunner interface, provider-neutral BuildStatus lifecycle, and how the orchestrator wires it into the build stage
+
+## Stovepipe
+
+- [Stovepipe Workflow](stovepipe/workflow.md) - Post-merge trunk-validation pipeline: ingest trunk push events (webhook + fallback poll), batch since last green, build to validate, record per-commit health, bisect to the offending commit, hand off to a remediation extension