ci: bootstrap pin moves to 2026.9.2.1

The release is published, mirrored and indexed: `latest` in
openxlings/xim-pkgindex points at 2026.9.2.1, and its `Publish Index Artifact`
run is green on that commit, which is what `xlings install` consumes.

The pin is bumped only after that sequence, never before it — CI installs the
pinned version from the index, so a pin ahead of the index is a job that
cannot start.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.1.1"
+    "mcpp": "2026.9.2.1"
   }
 }