feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

.agent/skills/bmad-create-epics-and-stories/steps/step-04-final-validation.md

@@ -90,6 +90,12 @@ Review the complete epic and story breakdown to ensure EVERY FR is covered:
 - Dependencies flow naturally
 - Foundation stories only setup what's needed
 - No big upfront technical work
+- **File Churn Check:** Do multiple epics repeatedly modify the same core files?
+  - Assess whether the overlap pattern suggests unnecessary churn or is incidental
+  - If overlap is significant: Validate that splitting provides genuine value (risk mitigation, feedback loops, context size limits)
+  - If no justification for the split: Recommend consolidation into fewer epics
+  - ❌ WRONG: Multiple epics each modify the same core files with no feedback loop between them
+  - ✅ RIGHT: Epics target distinct files/components, OR consolidation was explicitly considered and rejected with rationale
 
 ### 5. Dependency Validation (CRITICAL)