feat: refine ce:ideate skill with per-agent volume model and cross-cutting synthesis

- Clarify sub-agent volume: each agent targets ~10 ideas (40-60 raw, ~30-50 after dedupe)
- Reframe ideation lenses as starting biases, not constraints, to encourage cross-cutting ideas
- Add orchestrator synthesis step between merge/dedupe and critique
- Improve skill description with specific trigger phrases for better auto-discovery
- Update argument-hint to be user-facing ("feature, focus area, or constraint")
- Position ideate as optional entry point in workflow diagram, not part of core loop
- Update plugin metadata and README with new skill counts and descriptions

.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 28 specialized agents and 41 skills.",
+      "description": "AI-powered development tools that get smarter with every use. Make each unit of engineering work easier than the last. Includes 28 specialized agents and 47 skills.",
       "version": "2.40.0",
       "author": {
         "name": "Kieran Klaassen",