feat(stovepipe): add the build stage controller

[roychying]

Why?

Stovepipe needs a controller that actually triggers builds using the BuildRunner contract.

What?

Adds stovepipe/controller/build. Process unmarshals a BuildRequest (request id only), reloads the Request, no-ops if already terminal, resolves the runner via Factory.For(Config{QueueName}), reads the already-decided BuildStrategy/BaseURI (retryable if not yet visible — never re-derives full-vs-incremental itself), triggers via buildRunner.Trigger(ctx, baseURI, request.URI, nil), persists a Build row (BuildStore.Create, tolerant of ErrAlreadyExists), and publishes the resulting build id to the buildsignal topic, partitioned by build id.

Test Plan

stovepipe/controller/build/build_test.go covers: terminal-request no-op, unresolved build strategy (retryable), Factory.For/Trigger/BuildStore.Create/publish failure paths, and the happy path for both full and incremental strategies. go build ./... and go test ./stovepipe/controller/build/... pass.

Issue

[behinddwalls]

🛑 Automatic stack rebase aborted — diff mismatch

The rebase of chenghan.ying/stovepipe-build-controller completed without conflicts, but the resulting code diff does not match the original. This means the rebase silently altered code content. The branch was not force-pushed.

Please rebase manually and verify the changes are correct.