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/git-board/js/main.js

@@ -168,7 +168,7 @@
                              onSelect=${function (o, r) { setOwner(o); setRepo(r); }} />
             </div>
             <div class="gb-header__right">
-              <button class="btn-small" onClick=${loadBoard} disabled=${loading}>
+              <button class="sw-btn sw-btn--secondary sw-btn--sm" onClick=${loadBoard} disabled=${loading}>
                 ${loading ? '↻' : 'Refresh'}
               </button>
             </div>
@@ -196,7 +196,7 @@
           <p>Ask your admin to add a <strong>Gitea</strong> connection in
              <strong>Admin → Connections</strong>, or add a personal one in
              <strong>Settings → Connections</strong>.</p>
-          <button class="btn-small btn-primary"
+          <button class="sw-btn sw-btn--primary sw-btn--sm"
                   onClick=${function () { window.location.href = base + '/settings'; }}>
             Open Settings
           </button>
@@ -449,11 +449,11 @@
                             if (e.key === 'Enter' && (e.metaKey || e.ctrlKey)) postComment();
                           }} />
                 <div class="gb-modal__actions">
-                  <button class="btn-small btn-primary" disabled=${posting || !comment.trim()}
+                  <button class="sw-btn sw-btn--primary sw-btn--sm" disabled=${posting || !comment.trim()}
                           onClick=${postComment}>
                     ${posting ? 'Posting…' : 'Comment'}
                   </button>
-                  <button class="btn-small ${issue.state === 'open' ? 'btn-danger' : 'btn-secondary'}"
+                  <button class="sw-btn sw-btn--secondary sw-btn--sm ${issue.state === 'open' ? 'sw-btn sw-btn--danger sw-btn--md' : 'sw-btn sw-btn--secondary sw-btn--md'}"
                           onClick=${toggleState}>
                     ${issue.state === 'open' ? 'Close Issue' : 'Reopen Issue'}
                   </button>