refactor: one Go module; brain search in bin/brain + internal/brain. (#8)

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/rank/query.go

@@ -0,0 +1,9 @@
+package rank
+
+// BM25 ranks best-first, so the top hits are the *highest* scores; cosine
+// distance ranks best-first ascending. Both mirror kblib.py.
+const FTSStmt = "CALL QUERY_FTS_INDEX('Leaf', 'id', $q) " +
+	"RETURN node.id, node.text, node.root, node.source, score ORDER BY score DESC LIMIT $n"
+
+const VecStmt = "CALL QUERY_VECTOR_INDEX('Leaf', 'Leaf_vec', $q, $n) " +
+	"RETURN node.id, node.text, node.root, node.source, distance ORDER BY distance LIMIT $n"