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>

docs/EXTENSION-CSS.md

@@ -0,0 +1,149 @@
+# Extension CSS Contract
+
+> **Version**: v0.6.12 — Extension CSS Isolation
+
+This document defines the CSS contract between the Armature kernel and extension
+packages. Extensions **must** follow these rules; the kernel guarantees the listed
+classes and variables are stable public API.
+
+---
+
+## Naming Rule
+
+All class selectors in extension CSS (`packages/{slug}/css/main.css`) must start
+with `.ext-{slug}-`. The `{slug}` is the package directory name.
+
+```css
+/* Good */
+.ext-my-app-sidebar { ... }
+.ext-my-app-card { ... }
+
+/* Bad — will be rejected by the linter */
+.sidebar { ... }
+.my-sidebar { ... }
+```
+
+**Compound selectors**: Descendant classes scoped under your `.ext-{slug}` root
+are allowed to reference kernel classes or state modifiers:
+
+```css
+/* Allowed — kernel class scoped under extension namespace */
+.ext-my-app .sw-btn { margin-top: 8px; }
+
+/* Allowed — state modifier on an extension element */
+.ext-my-app-item.active { ... }
+```
+
+Run `bash scripts/lint-package-css.sh` to validate. The linter checks that the
+**first** class selector in every rule starts with `.ext-{slug}`.
+
+---
+
+## Stable Kernel Classes
+
+Extensions may reference these classes in compound selectors. They are part of
+the public API and will not change without a major version bump.
+
+### Components (from `sw-primitives.css`)
+
+| Class pattern | Component |
+|---------------|-----------|
+| `.sw-btn`, `.sw-btn--{variant}`, `.sw-btn--{size}` | Buttons |
+| `.sw-input` | Text inputs |
+| `.sw-field`, `.sw-field__label`, `.sw-field__hint` | Form fields |
+| `.sw-dialog`, `.sw-dialog__header`, `.sw-dialog__body`, `.sw-dialog__footer` | Dialogs |
+| `.sw-toast`, `.sw-toast-container` | Toast notifications |
+| `.sw-menu`, `.sw-menu-item` | Context menus |
+| `.sw-tabs`, `.sw-tab-btn` | Tab strips |
+| `.sw-dropdown` | Custom dropdowns |
+| `.sw-spinner` | Loading spinners |
+| `.sw-avatar` | User avatars |
+| `.sw-drawer` | Slide-out drawers |
+| `.sw-banner` | Banner bars |
+| `.sw-tooltip` | Tooltips |
+
+### Extension Mount
+
+The extension surface container has a `data-ext` attribute set to the package
+slug. Use this for scoping if needed:
+
+```css
+[data-ext="my-app"] .ext-my-app-sidebar { ... }
+```
+
+---
+
+## Stable CSS Variables
+
+All variables from `variables.css` are public API. Extensions should use these
+instead of hardcoded colors to respect the user's theme.
+
+### Colors
+
+| Variable | Purpose |
+|----------|---------|
+| `--bg` | Page background |
+| `--bg-secondary` | Secondary/darker background |
+| `--bg-elevated` | Elevated surface background |
+| `--bg-raised` | Raised card background |
+| `--bg-surface` | Surface-level background |
+| `--bg-hover` | Hover state background |
+| `--bg-active` | Active/pressed state background |
+| `--bg-code` | Code block background |
+| `--text` | Primary text color |
+| `--text-2` | Secondary text color |
+| `--text-3` | Tertiary/muted text color |
+| `--text-on-color` | Text on colored backgrounds |
+| `--accent` | Primary accent color |
+| `--accent-dim` | Dimmed accent for backgrounds |
+| `--accent-hover` | Accent hover state |
+| `--accent-light` | Light accent variant |
+| `--border` | Default border color |
+| `--border-light` | Light border variant |
+| `--border-elevated` | Border for elevated surfaces |
+| `--danger` | Error/destructive color |
+| `--danger-dim` | Dimmed danger background |
+| `--danger-light` | Light danger variant |
+| `--success` | Success/positive color |
+| `--success-dim` | Dimmed success background |
+| `--success-light` | Light success variant |
+| `--warning` | Warning/caution color |
+| `--warning-dim` | Dimmed warning background |
+| `--warning-light` | Light warning variant |
+| `--purple` | Purple accent |
+| `--purple-dim` | Dimmed purple background |
+
+### Layout & Typography
+
+| Variable | Purpose |
+|----------|---------|
+| `--font` | Primary font family |
+| `--mono` | Monospace font family |
+| `--radius` | Default border-radius (8px) |
+| `--radius-lg` | Large border-radius (12px) |
+| `--transition` | Default transition timing |
+| `--shadow-lg` | Large elevation shadow |
+| `--overlay` | Modal overlay color |
+| `--glass` | Glassmorphism backdrop |
+| `--input-bg` | Form input background |
+| `--sidebar-w` | Sidebar width |
+
+---
+
+## What Is Internal
+
+Everything not listed above is **internal kernel CSS** and may change between
+minor versions. Extensions must not depend on:
+
+- Kernel layout classes (`.admin-*`, `.surface-*`, `.sidebar`, etc.)
+- Kernel CSS file load order
+- Specific HTML structure of the shell or topbar
+- Undocumented CSS variables
+
+---
+
+## Enforcement
+
+The linter script `scripts/lint-package-css.sh` runs against all
+`packages/*/css/main.css` files. It exits non-zero if any rule's first class
+selector does not start with `.ext-{slug}`.