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/metadata.json

@@ -0,0 +1,9 @@
+{
+  "uuid": "dashward@hangman-lab.top",
+  "name": "Dashward",
+  "description": "A dedicated rightmost workspace for custom widgets, in the spirit of macOS Dashboard.",
+  "shell-version": ["48"],
+  "url": "https://git.hangman-lab.top/hzhang/Dashward",
+  "version": 1,
+  "settings-schema": "org.gnome.shell.extensions.dashward"
+}