chore: P0 skeleton

Bootstrap the Dashward repo per arch/UBUNTU-DASHBOARD-SPACE.md:

- pnpm-workspaces monorepo (sdk, extension, container, widgets-builtin/*)
- GNOME extension stub (metadata.json, src/*.ts placeholders for warden,
  guard, supervisor, entry UX, DBus service)
- WebKit container stub (GJS main + page-side runtime + dashboard.html)
- TypeScript widget SDK (defineWidget + types)
- Builtin clock widget as the first SDK consumer example
- DBus interface XML (proto/shell.iface.xml) and shared types
- esbuild configs for extension and container; tsc for SDK
- Design doc copied in at repo root for discoverability

No functional logic yet -- all components are placeholders that compose
in extension.ts so the build chain can be exercised. P1 (workspace
warden) starts next.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sdk/src/types.ts

@@ -0,0 +1,41 @@
+export type Theme = 'light' | 'dark';
+
+export interface WidgetHost {
+  readonly element: HTMLElement;
+  readonly instanceId: string;
+}
+
+export interface ConfigApi<T> {
+  get(): T;
+  set(value: Partial<T>): Promise<void>;
+  onChange(handler: (value: T) => void): () => void;
+}
+
+export interface SystemApi {
+  getTheme(): Theme;
+  onThemeChange(handler: (theme: Theme) => void): () => void;
+}
+
+export interface LifecycleApi {
+  onMount(handler: () => void): void;
+  onUnmount(handler: () => void): void;
+  onVisibilityChange(handler: (visible: boolean) => void): () => void;
+}
+
+export interface ShellApi {
+  call<T = unknown>(method: string, args?: unknown): Promise<T>;
+}
+
+export interface WidgetContext<T> {
+  config: ConfigApi<T>;
+  system: SystemApi;
+  lifecycle: LifecycleApi;
+  shell: ShellApi;
+}
+
+export interface WidgetDefinition<T> {
+  id: string;
+  defaultConfig: T;
+  mount(host: WidgetHost, ctx: WidgetContext<T>): void;
+  configUI?(host: WidgetHost, ctx: WidgetContext<T>): void;
+}