feat: write leafs incrementally without rebuilding the graph.

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/SKILL.md

@@ -26,6 +26,7 @@ second independent source when local roots cannot confirm. An answer is
 bin/brain/search.go "Matrix federation"                # pointers + snippets, YAML
 bin/brain/search.go "onlyoffice postgres" --root facts # restrict to confirmed
 bin/brain/search.go "where is cs-lexicon" --json | yq '.[].ref'
+bin/brain/add.go --text T --root facts --source "a.md x b.md"
 bin/brain/get.go <id> --body                           # full chunk only when needed
 bin/brain/stats.go                                     # index health
 bin/brain/eval.go                                      # recall@5 >= 0.95 gate (Go; Python bin/kb/eval is CI fallback)

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