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/dashboard/js/main.js

@@ -343,8 +343,8 @@
             (ch.description ? '<div style="font-size:12px;color:var(--text-2);margin-bottom:8px;"><strong>Description:</strong> ' + (typeof esc === 'function' ? esc(ch.description) : ch.description) + '</div>' : '') +
             (ch.created_at ? '<div style="font-size:12px;color:var(--text-3);margin-bottom:16px;">Created: ' + new Date(ch.created_at).toLocaleString() + '</div>' : '') +
             '<div style="display:flex;justify-content:flex-end;gap:8px;">' +
-                '<button class="btn-small" id="dashDetailClose">Close</button>' +
-                '<button class="btn-small btn-primary" id="dashDetailOpen">Open Chat</button>' +
+                '<button class="sw-btn sw-btn--secondary sw-btn--sm" id="dashDetailClose">Close</button>' +
+                '<button class="sw-btn sw-btn--primary sw-btn--sm" id="dashDetailOpen">Open Chat</button>' +
             '</div>';
 
         modal.appendChild(box);