v0.2.1 — host queries: list_entities() + edges_since()

• host queries (requested by the first production consumer for its
  intelligence layer): Memory.list_entities() — distinct ids with
  edge/episode counts, for proactive-recall planning and coverage audits — and
  Memory.edges_since(user_id, since) — edges learned after a date, filtered
  on provenance.observed_at, including superseded/quarantined material so
  change-detection sees everything. Host/admin surface; neither is an MCP tool
  (cross-user enumeration is not an agent capability). Store gains
  list_users() (non-abstract, like forget_user).