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

README.md

@@ -184,17 +184,20 @@ Notes:
 
 ```
 Brainstorm → Plan → Work → Review → Compound → Repeat
+    ↑
+  Ideate (optional — when you need ideas)
 ```
 
 | Command | Purpose |
 |---------|---------|
+| `/ce:ideate` | Discover high-impact project improvements through divergent ideation and adversarial filtering |
 | `/ce:brainstorm` | Explore requirements and approaches before planning |
 | `/ce:plan` | Turn feature ideas into detailed implementation plans |
 | `/ce:work` | Execute plans with worktrees and task tracking |
 | `/ce:review` | Multi-agent code review before merging |
 | `/ce:compound` | Document learnings to make future work easier |
 
-The `/ce:brainstorm` skill supports collaborative dialogue to clarify requirements and compare approaches before committing to a plan.
+The `/ce:ideate` skill proactively surfaces strong improvement ideas, and `/ce:brainstorm` then clarifies the selected one before committing to a plan.
 
 Each cycle compounds: brainstorms sharpen plans, plans inform future plans, reviews catch more issues, patterns get documented.