feat(stovepipe): add Build entity and terminal Request states

[roychying]

Why?

Stovepipe's build/buildsignal pipeline stage needs a Build entity to track a triggered build's lifecycle, and buildsignal/DLQ reconciliation need a way to know when a Request has reached a state the pipeline will never advance past.

What?

• Adds stovepipe/entity/build.go:
  • BuildStatus (accepted/running/succeeded/failed/cancelled) with IsTerminal() — shaped like SubmitQueue's BuildStatus but defined locally rather than shared.
  • Build — a single triggered build (ID, RequestID, URI, BaseURI, Status, Version), immutable except Status/Version, which only buildsignal writes after build creates the row.
  • BuildID — a lightweight wrapper for the runner-assigned id, used on the queue and as the BuildRunner Status/Cancel parameter.
  • BuildMetadata — caller-supplied, provider-echoed free-form metadata (empty for now).
  • JSON (de)serialization helpers (ToBytes/*FromBytes) for queue payloads on all three types.
• Extends stovepipe/entity/request.go:
  • Two new RequestState values: recorded_green / recorded_not_green, written by record. once whole-repo greenness is written for a URI (normal path or fail-closed reconciler).
  • RequestState.IsTerminal() — true for superseded or either recorded outcome — used by buildsignal to short-circuit polling once a Request is already done.

Test Plan

go test ./stovepipe/... passes locally.
Need to conduct e2e testing after filling in more logics.

Issue

[CLAassistant]

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