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>

CHANGELOG.md

@@ -2,6 +2,37 @@
 
 All notable changes to Armature are documented here.
 
+## 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.
+
+### Added
+
+- **`data-ext` attribute** on extension mount container — enables scoped
+  selectors like `[data-ext="chat"] .ext-chat-app`.
+- **CSS linter** (`scripts/lint-package-css.sh`) — validates that the first
+  class selector in every extension CSS rule starts with `.ext-{slug}`.
+  Exempts `:root`, `@keyframes`, `@font-face`, `@media`, kernel `.sw-*`
+  classes, and CodeMirror `.cm-*` classes.
+- **Kernel CSS contract** (`docs/EXTENSION-CSS.md`) — documents stable
+  public classes and CSS variables that extensions may reference. Everything
+  else is internal kernel CSS.
+
+### Changed
+
+- **12 packages migrated** — all class selectors renamed to `.ext-{slug}-*`:
+  chat, dashboard, editor, git-board, hello-dashboard, icd-test-runner,
+  notes, schedules, sdk-test-runner, tasks, team-activity-log, workflow-demo.
+  CSS and JS files updated in lockstep.
+- **`icd-test-runner`** — ID selectors (`#extension-mount`) converted to
+  class-based selectors with proper prefix.
+- **`editor` cross-references** — compound selectors referencing notes
+  classes updated to new `.ext-notes-*` names.
+- **`chat` kernel overrides** — `.sw-dialog:has(...)` override scoped under
+  `[data-ext="chat"]` instead of global.
+
 ## v0.6.11 — CSS Deduplication
 
 One class per concept. The old `primitives.css` button, toast, popup-menu,