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>

proto/shell.iface.xml

@@ -0,0 +1,38 @@
+<!-- DBus interface for the Dashward Shell extension.
+     Bus name: top.hangmanlab.Dashward.Shell
+     Object path: /top/hangmanlab/Dashward
+     See design §8.1. -->
+<node>
+  <interface name="top.hangmanlab.Dashward.Shell">
+
+    <method name="Ready"/>
+
+    <method name="RequestEnterDashboard"/>
+
+    <method name="RequestExitDashboard"/>
+
+    <method name="GetSystemInfo">
+      <arg type="a{sv}" direction="out" name="info"/>
+    </method>
+
+    <method name="WidgetIPC">
+      <arg type="s" direction="in" name="widgetId"/>
+      <arg type="s" direction="in" name="method"/>
+      <arg type="v" direction="in" name="args"/>
+      <arg type="v" direction="out" name="result"/>
+    </method>
+
+    <signal name="ThemeChanged">
+      <arg type="s" name="theme"/>
+    </signal>
+
+    <signal name="EditModeRequested">
+      <arg type="b" name="enter"/>
+    </signal>
+
+    <signal name="SystemInfoChanged">
+      <arg type="a{sv}" name="changed"/>
+    </signal>
+
+  </interface>
+</node>