D24 — Automatic keyword index update after concept_write

Decision. toolConceptWrite receives a **search.Index and, after each successful write, calls (*idx).Add(id, content) to update the in-memory index incrementally. The server no longer requires index_rebuild after each write. The post-write read error is silent (the write is already confirmed). Rationale. The agent should not have had to trigger index_rebuild manually: the correct behavior is that search immediately reflects what was written. The incremental update (single add) is O(n-terms) instead of the full rebuild's O(n-concepts). Exception: with RegisterKBToolsWithEmbed active, search uses a separate index (newIdx) that is not updated by concept_write — index_rebuild remains necessary in that path. Exception superseded by D36: the index is now shared (liveIndex) across all paths and concept_write also updates the persisted FTS5.