fix: review fixes — stale refs, skill counts, and validation guidance

- Fix -plan.md → -beta-plan.md in ce:plan-beta post-generation question
- Remove stale brainstorm doc reference from solutions doc
- Update plugin.json and marketplace.json skill counts (42 → 44)
- Add generic beta skill validation guidance to AGENTS.md and solutions doc

.claude-plugin/marketplace.json

@@ -11,7 +11,7 @@
   "plugins": [
     {
       "name": "compound-engineering",
-      "description": "AI-powered development tools that get smarter with every use. Make each unit of engineering work easier than the last. Includes 29 specialized agents and 42 skills.",
+      "description": "AI-powered development tools that get smarter with every use. Make each unit of engineering work easier than the last. Includes 29 specialized agents and 44 skills.",
       "version": "2.41.0",
       "author": {
         "name": "Kieran Klaassen",