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>

extension/widgets-builtin/clock/widget.json

@@ -0,0 +1,10 @@
+{
+  "id": "clock",
+  "name": "Clock",
+  "version": "0.0.0",
+  "entry": "dist/index.js",
+  "icon": "icon.svg",
+  "size": { "w": 2, "h": 2, "minW": 1, "minH": 1, "maxW": 4, "maxH": 4 },
+  "permissions": ["timer"],
+  "configSchema": { "hour12": "boolean", "showSeconds": "boolean" }
+}