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/sections/manifest.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://gstack.dev/schemas/section-manifest.json",
+  "skill": "design-shotgun",
+  "version": 1,
+  "note": "PASSIVE registry (v2 plan T9 / CM2). Fields are IDs, file paths, human titles, and human-readable trigger text ONLY. The skeleton's decision-tree prose is the ONLY place that decides WHEN to read a section; required-reads live in the E2E fixtures. No machine predicate here — see docs/designs/v2_PLAN.md:663.",
+  "sections": [
+    {
+      "id": "doctrine",
+      "file": "doctrine.md",
+      "title": "UX principles doctrine (how users actually behave)",
+      "trigger": "writing variant concepts or design briefs (Step 3 onward) — the UX-principles doctrine governs every design direction"
+    }
+  ]
+}