refactor(skills): fold brainstorming Key Principles into points of use

Five of six principles restated the Checklist and Process sections
verbatim-in-spirit. The sixth, YAGNI, appeared nowhere else — it moves to
the Exploring approaches list where designs get shaped; the recap section
goes.

skills/brainstorming/SKILL.md

@@ -77,6 +77,7 @@ digraph brainstorming {
 - Propose 2-3 different approaches with trade-offs
 - Present options conversationally with your recommendation and reasoning
 - Lead with your recommended option and explain why
+- YAGNI ruthlessly - remove unnecessary features from every approach and design
 
 **Presenting the design:**
 
@@ -130,15 +131,6 @@ Wait for the user's response. If they request changes, make them and re-run the
 - Invoke the writing-plans skill to create a detailed implementation plan
 - Do NOT invoke any other skill. writing-plans is the next step.
 
-## Key Principles
-
-- **One question at a time** - Don't overwhelm with multiple questions
-- **Multiple choice preferred** - Easier to answer than open-ended when possible
-- **YAGNI ruthlessly** - Remove unnecessary features from all designs
-- **Explore alternatives** - Always propose 2-3 approaches before settling
-- **Incremental validation** - Present design, get approval before moving on
-- **Be flexible** - Go back and clarify when something doesn't make sense
-
 ## Visual Companion
 
 A browser-based companion for showing mockups, diagrams, and visual options during brainstorming. Available as a tool — not a mode. Accepting the companion means it's available for questions that benefit from visual treatment; it does NOT mean every question goes through the browser.