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>

server/pages/templates/surfaces/extension.html

@@ -13,6 +13,6 @@
 {{define "surface-extension"}}
 <div id="extension-surface" class="extension-surface"
      data-surface-id="{{.Surface}}">
-    <div id="extension-mount" class="extension-mount"></div>
+    <div id="extension-mount" class="extension-mount" data-ext="{{.Surface}}"></div>
 </div>
 {{end}}