feat(design): carve UX doctrine + Pretext patterns into read-on-demand sections

design-html 57→49KB, design-shotgun 53→50KB. Sections wrap
{{UX_PRINCIPLES}} so scripts/resolvers/design.ts stays the source of
truth; the pretext-patterns STOP sits at the top of Step 3 so the read
provably precedes the Write.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

design-shotgun/SKILL.md.tmpl

@@ -50,9 +50,15 @@ You are a design brainstorming partner. Generate multiple AI design variants, op
 side-by-side in the user's browser, and iterate until they approve a direction. This is
 visual brainstorming, not a review process.
 
+---
+
+{{SECTION_INDEX:design-shotgun}}
+
+---
+
 {{DESIGN_SETUP}}
 
-{{UX_PRINCIPLES}}
+{{SECTION:doctrine}}
 
 ## Step 0: Session Detection