feat: in-process HTTP search; /get /stats /audit /ingest. (#13)

bin/brain/serve.go (ladybug tags) calls internal/brain instead of exec.
HTTP tests inject a fake API so CI stays cgo-free. ExecSearcher remains
the fallback when the binary is built without system_ladybug.

bin/serve.go

@@ -18,5 +18,5 @@ func main() {
 			os.Setenv("KB_ROOT", wd)
 		}
 	}
-	httpapi.Run()
+	httpapi.Run(nil)
 }