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.

internal/brain/rank/evalq.go

@@ -0,0 +1,16 @@
+package rank
+
+// Eval control questions (recall@5). Kept here so CI can test the gate
+// table without ladybug cgo. The runner lives in internal/brain (cgo).
+const EvalRecallThreshold = 0.95
+
+type EvalQuestion struct {
+	Query    string
+	Fragment string
+}
+
+var EvalQuestions = []EvalQuestion{
+	{"hybrid search fts and vector", "BM25"},
+	{"eslider devops engineer", "DevOps"},
+	{"ladybugdb graph engine storage", "LadybugDB"},
+}