Feat v0.7.3 extension shell migration

Migrate Chat, Notes, and Schedules from legacy sw.shell.Topbar to the
v0.7.0 shell topbar contract (sw.shell.topbar.setTitle/setSlot),
eliminating double topbars on all extension surfaces.

- Chat v0.3.0: reactive slot for thread title + People button
- Notes v0.9.0: slot for New/Import/Graph buttons
- Schedules v0.2.0: reactive slot for count + New button
- 6 new shell-topbar runner tests (2 per surface)
- Headless E2E deferred to v0.7.5

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/notes/manifest.json

@@ -6,7 +6,7 @@
   "route": "/s/notes",
   "auth": "authenticated",
   "layout": "single",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "icon": "📝",
   "description": "Markdown notes surface with rich editor, folders, tags, backlinks, import/export, sidebar tabs, document outline, and note graph.",
   "author": "armature",