D43 — Automatic SQLite index rebuild at startup if empty, without embedding

Decision. <kb>/.cartographer/index.db is gitignored and derived: after a fresh clone (e.g. k8s pod restart) it starts empty and search answered count=0 until someone called index_rebuild manually. At startup, for each mounted KB, if ix.Count() == 0 the server rebuilds FTS5 from the .md files (mcpserver.EnsureSQLIndexFresh, reuses rebuildSQLIndex), deliberately without embedding. Best-effort: an error logs and does not prevent startup. Rationale. No embedding at boot because Ollama may be slow/absent — blocking startup on an optional external dependency would be the wrong coupling; the cache repopulates anyway at the first index_rebuild/semantic search. COUNT(*)==0 is the cheapest signal and consistent with the "derived and rebuildable index" invariant. Details: docs/control-plane.md §Search index.