Feat v0.6.11 CSS deduplication (#46)

Retire old primitives.css button/toast/popup-menu/dropdown/tabs systems.
sw-primitives.css is now the single source of truth.

- Migrate 29 files from .btn-* to .sw-btn .sw-btn--{variant} .sw-btn--{size}
- Delete old toast, popup-menu, dropdown, tabs CSS (no JS references)
- Resolve .sw-dropdown and .sw-tabs class name collisions
- Resolve .settings-section duplicate (modals.css removed, surfaces.css authoritative)
- Add .sw-btn--success variant, --bg-active CSS variable
- Fix sdk-test-runner wrong variable names, icd-test-runner button fallbacks
- Add scripts/audit-css-collisions.sh

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/workflow-demo/css/main.css

@@ -213,17 +213,7 @@
     cursor: pointer;
     font-weight: 500;
 }
-.demo-card .btn-primary {
-    background: var(--accent);
-    color: var(--text-on-color);
-}
-.demo-card .btn-primary:hover {
-    background: var(--accent-hover);
-}
-.demo-card .btn-primary:disabled {
-    opacity: 0.5;
-    cursor: default;
-}
+/* demo-card buttons — uses kernel .sw-btn--primary */
 
 /* Public link row */
 .public-link-row {