feat: brain get/stats/eval call internal/brain, not Python (#18)

Read path is cgo like search. Control questions live in rank so CI can
test them without ladybug. Python bin/kb/{get,stats,eval} stays the
runner fallback.

docs/design.md

@@ -61,4 +61,12 @@ corpus HEAD.
 - C: narrative — READMEs, AGENTS.md, docs
 
 Confirmed = A×B or B×C agreement. Single source = hypothesis + `(not confirmed)`.
-Conflicting pairings (≥2 yes vs ≥2 no) = hypothesis (OQ1 → v2 resolution).
+Conflicting pairings (≥2 yes vs ≥2 no) = hypothesis (OQ1 → v2 resolution).
+
+## Read path
+
+`bin/brain/get.go`, `stats.go`, and `eval.go` call `internal/brain` with cgo
+(`system_ladybug`). They do not exec Python. Control questions for recall@5
+live in `internal/brain/rank` so CI can test the table without libladybug.
+Python `bin/kb/{get,stats,eval}` remain for GitHub Actions until the runner
+has ladybug cgo. Index/write is still `bin/kb/index`.