chore: bump bootstrap mcpp to 0.0.20, add release skill

- Update .xlings.json mcpp pin from 0.0.17 to 0.0.20
- Add .agents/skills/mcpp-release/ documenting the full release
  workflow, including the critical two-file version sync requirement

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "0.0.17"
+    "mcpp": "0.0.20"
   }
 }