Feat v0.6.12 extension css isolation

Prefix enforcement prevents extension CSS from leaking into the kernel
or sibling extensions. All 12 in-tree packages migrated to .ext-{slug}-*
naming convention.

- Add data-ext attribute to extension mount container
- Add CSS linter (scripts/lint-package-css.sh) enforcing .ext-{slug} prefix
- Add kernel CSS contract doc (docs/EXTENSION-CSS.md)
- Migrate 12 packages: chat, dashboard, editor, git-board, hello-dashboard,
  icd-test-runner, notes, schedules, sdk-test-runner, tasks,
  team-activity-log, workflow-demo (CSS + JS in lockstep)

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

packages/icd-test-runner/css/main.css

@@ -1,38 +1,38 @@
 /* ICD Test Runner — Surface Styles */
 
-#extension-mount {
+.ext-icd-test-runner-root {
   font-family: var(--font, 'DM Sans', sans-serif);
 }
 
-#extension-mount h1 {
+.ext-icd-test-runner-root h1 {
   font-family: var(--font, 'DM Sans', sans-serif);
   letter-spacing: -0.3px;
 }
 
-#extension-mount table {
+.ext-icd-test-runner-root table {
   font-variant-numeric: tabular-nums;
 }
 
-#extension-mount table td,
-#extension-mount table th {
+.ext-icd-test-runner-root table td,
+.ext-icd-test-runner-root table th {
   border-color: var(--border);
 }
 
-#extension-mount table tbody tr:last-child {
+.ext-icd-test-runner-root table tbody tr:last-child {
   border-bottom: none;
 }
 
-#extension-mount table tbody tr:hover {
+.ext-icd-test-runner-root table tbody tr:hover {
   background: var(--bg-hover) !important;
 }
 
 /* Buttons — uses kernel sw-btn system, minor overrides for weight */
-#extension-mount .sw-btn {
+.ext-icd-test-runner-root .sw-btn {
   font-weight: 600;
 }
 
 /* Status dot animation for running state */
-@keyframes pulse-dot {
+@keyframes ext-icd-test-runner-pulse-dot {
   0%, 100% { opacity: 1; }
   50% { opacity: 0.4; }
 }