feat: opt-in LLM reranking and ingest analysis (disabled by default)

[salishforge]

Optional LLM-powered features activated via ENABLE_LLM_RERANK and ENABLE_LLM_INGEST env vars. Both disabled by default (zero cost). Reranking sends top-20 results to LLM for relevance ordering. Ingest analysis classifies memory type and extracts entities.