QA: replace qa-lab-runtime with qa-runtime

Introduce a tiny generic qa-runtime seam for shared live-lane helpers and
repoint qa-matrix to it. This keeps the qa-lab host split while removing
the host-owned runtime name from runner code.

Drop the old qa-lab-runtime shim/export now that nothing consumes it and
keep the plugin-sdk surface aligned with the new seam.

scripts/lib/plugin-sdk-entrypoints.json

@@ -179,7 +179,7 @@
   "matrix-runtime-surface",
   "matrix-surface",
   "matrix-thread-bindings",
-  "qa-lab-runtime",
+  "qa-runtime",
   "qa-runner-runtime",
   "mattermost",
   "mattermost-policy",