Strengthen brainstorming skill trigger and add skill priority guidance

- Update brainstorming description to use imperative "You MUST use this"
- Clarify trigger: "before any creative work - creating features, building
  components, adding functionality, or modifying behavior"
- Add Skill Priority section to using-superpowers to ensure process skills
  (brainstorming, debugging) are invoked before implementation skills

skills/using-superpowers/SKILL.md

@@ -57,6 +57,16 @@ These thoughts mean STOP—you're rationalizing:
 | "I'll just do this one thing first" | Check BEFORE doing anything. |
 | "This feels productive" | Undisciplined action wastes time. Skills prevent this. |
 
+## Skill Priority
+
+When multiple skills could apply, use this order:
+
+1. **Process skills first** (brainstorming, debugging) - these determine HOW to approach the task
+2. **Implementation skills second** (frontend-design, mcp-builder) - these guide execution
+
+"Let's build X" → brainstorming first, then implementation skills.
+"Fix this bug" → debugging first, then domain-specific skills.
+
 ## Skill Types
 
 **Rigid** (TDD, debugging): Follow exactly. Don't adapt away discipline.