feat: write leafs incrementally without rebuilding the graph. (#29)

Ladybug 0.19 stays FTS/HNSW queryable on MERGE of new ids; DROP INDEX
was the fatal path. bin/brain/add.go and POST /ingest land facts+info
in one transaction so watch/mail/git can become leafs now (Gitea #14).

skills/brain/tools.md

@@ -8,4 +8,4 @@ Serve: `bin/brain/serve.go` (`GET /openapi.json`, `POST /mcp`).
 - `get` — read one leaf by id
 - `stats` — index health
 - `audit` — facts confidence histogram
-- `ingest` — rebuild hint (write is v2)
+- `ingest` — add a leaf without rebuild