feat(hyperguild): brain query subcommand

Adds 'hyperguild brain query <topic>' against the brain HTTP REST
/query endpoint. Default human output prints path + score + title;
--json passes through the response envelope. --limit overrides the
default 5-result cap.

runBrainWrite remains a stub for Task 5.

cmd/hyperguild/main.go

@@ -26,7 +26,7 @@ func notYet(ctx context.Context, args []string, stdin io.Reader, stdout, stderr
 func subcommands() map[string]subcommand {
 	return map[string]subcommand{
 		"tier":  runTier,
-		"brain": notYet,
+		"brain": runBrain,
 		"mode":  notYet,
 	}
 }