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

plugins/compound-engineering/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "compound-engineering",
   "version": "2.41.0",
-  "description": "AI-powered development tools. 29 agents, 42 skills, 1 MCP server for code review, research, design, and workflow automation.",
+  "description": "AI-powered development tools. 29 agents, 44 skills, 1 MCP server for code review, research, design, and workflow automation.",
   "author": {
     "name": "Kieran Klaassen",
     "email": "kieran@every.to",