knowledge: cosine dedup, fact create endpoint, recall nudge

Phase 1 of the knowledge curation work. Three small server-side changes
to support an Apollo-side curation surface and reduce the agent's near-
duplicate output rate going forward:

- upsert_entity grows an embedding-cosine fallback after the exact name
  match misses. New entities whose embedding sits above
  ENTITY_DEDUP_COSINE_THRESHOLD (default 0.92) against any same-type
  active entity collapse onto the existing row. Eliminates the Sarah /
  Sara / Sarah J. trio the FTS5 prefix check was missing.
- POST /knowledge/facts symmetric with the existing PATCH/DELETE so the
  curation UI can create facts directly. Persona-scoped via X-Persona-Id;
  validates subject (and optional object) entity existence; reuses
  KnowledgeDao::upsert_fact so corroboration semantics match the agent
  path.
- One sentence in build_system_content telling the agent to call
  recall_entities before store_entity when a name resembles something
  already known. Cheap; complements the DAO-layer guard.

Includes upsert_entity_collapses_near_duplicate_by_embedding test
covering both the collapse-on-near-match path and the don't-collapse-on-
unrelated-embedding path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/insight_generator.rs

@@ -3204,6 +3204,7 @@ Return ONLY the summary, nothing else."#,
                — surrounding events matter even when a contact is known.\n\
              - Use recall_facts_for_photo + recall_entities to load any prior knowledge about subjects in the photo.\n\
              - When you identify people / places / events / things, use store_entity + store_fact to grow the persistent memory.\n\
+             - Before store_entity, call recall_entities to check whether a similar name already exists; reuse the existing entity_id rather than creating a near-duplicate (e.g. \"Sara\" vs \"Sarah J.\"). The DAO will collapse obvious cosine matches, but choosing the existing id keeps facts and photo links consolidated.\n\
              - A tool returning no results is informative; continue with the others.",
         );