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.

bin/brain/eval.go

@@ -1,20 +1,22 @@
-//usr/bin/env go run -tags=brain_eval "$0" "$@"; exit
-//go:build brain_eval
+//usr/bin/env go run -tags=system_ladybug,brain_eval "$0" "$@"; exit
+//go:build cgo && system_ladybug && brain_eval
 //
 // bin/brain/eval.go - recall@5 gate.
 //
 //	./bin/brain/eval.go
 //	./bin/brain/eval.go --json
 //
+// Needs CGO + libladybug. Python bin/kb/eval is the CI fallback (no cgo).
+// Control questions live in internal/brain/rank (cgo-free).
 // NOTE: never run `gofmt -w` on this file — it breaks the shebang.
 package main
 
 import (
 	"os"
 
-	"github.com/eSlider/2dph/internal/cmdbin"
+	"github.com/eSlider/2dph/internal/brain"
 )
 
 func main() {
-	os.Exit(cmdbin.ExecFile("bin/kb/eval", os.Args[1:]))
+	os.Exit(brain.MainEval(os.Args[1:]))
 }