QA: remove runner install fallback catalog

Drop the generated qa-runner catalog and the missing/install placeholder
path for repo-private QA runners. The host should discover bundled QA
commands from manifest plus runtime surface only.

Also trim stale qa-matrix install docs and package metadata so the
source-only QA policy stays consistent.

scripts/lib/qa-runner-catalog.json

@@ -1,8 +0,0 @@
-[
-  {
-    "pluginId": "qa-matrix",
-    "commandName": "matrix",
-    "description": "Run the Docker-backed Matrix live QA lane against a disposable homeserver",
-    "npmSpec": "@openclaw/qa-matrix"
-  }
-]