QA: drop dead qa-lab-runtime shim

Remove the old qa-lab-runtime shim now that qa-runtime is the only live
consumer seam. This leaves one tiny shared runtime facade instead of two
parallel names for the same private helper surface.

src/plugin-sdk/qa-lab-runtime.ts

@@ -1,39 +0,0 @@
-import { loadBundledPluginPublicSurfaceModuleSync } from "./facade-runtime.js";
-
-type QaLabRuntimeSurface = {
-  defaultQaRuntimeModelForMode: (
-    mode: string,
-    options?: {
-      alternate?: boolean;
-      preferredLiveModel?: string;
-    },
-  ) => string;
-  startQaLiveLaneGateway: (...args: unknown[]) => Promise<unknown>;
-};
-
-function isMissingQaLabRuntimeError(error: unknown) {
-  return (
-    error instanceof Error &&
-    (error.message === "Unable to resolve bundled plugin public surface qa-lab/runtime-api.js" ||
-      error.message.startsWith("Unable to open bundled plugin public surface "))
-  );
-}
-
-export function loadQaLabRuntimeModule(): QaLabRuntimeSurface {
-  return loadBundledPluginPublicSurfaceModuleSync<QaLabRuntimeSurface>({
-    dirName: "qa-lab",
-    artifactBasename: "runtime-api.js",
-  });
-}
-
-export function isQaLabRuntimeAvailable(): boolean {
-  try {
-    loadQaLabRuntimeModule();
-    return true;
-  } catch (error) {
-    if (isMissingQaLabRuntimeError(error)) {
-      return false;
-    }
-    throw error;
-  }
-}