Add design system, audit sections, apply global classes

Design system (design/):
- bcw-css-variables.json: color, spacing (--space-xs→2xl), typography (--h1→h6, --text-sm→xl)
- bcw-color-palette.json: named swatches referencing CSS variables
- bcw-global-classes.json: section-container, section-header, col-3, col-4, features-container, footer-sub-section
- design.md: full variable reference and usage guide

Templates:
- features-3col + features-4col: add global_classes embed + _cssGlobalClasses references
- All headers + footers: apply var(--color-primary) background to section elements
- footer-columns: add footer-sub-section class to column containers
- Remove content-image-left (duplicate of content-image-right with children reversed)
- testimonials: gap tweak 12px → 15px on card containers

Repo hygiene:
- Add .gitignore (Updated_Sections/)
- Clear Updated_Sections staging folder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

headers/header-centered.json

@@ -8,7 +8,13 @@
       "name": "section",
       "parent": 0,
       "children": ["con001"],
-      "settings": [],
+      "settings": {
+        "_background": {
+          "color": {
+            "raw": "var(--color-primary)"
+          }
+        }
+      },
       "label": "Header Centered"
     },
     {