refactor: one Go module; brain search in bin/brain + internal/brain.

Collapse nested kbsearch/chats go.mod into the root module. Ranking stays
cgo-free under internal/brain/rank so CI does not need ladybug. bin/kb/search
is a deprecation wrapper that still sets CGO and builds the binary.

internal/brain/types.go

@@ -0,0 +1,13 @@
+package brain
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/internal/brain/rank"
+)
+
+func eps() string { return os.Getenv("KBTEST_EPS") }
+
+// Hit is the search hit type; ranking lives in package rank so CI can test
+// it without the native ladybug library.
+type Hit = rank.Hit