fix: P1 plugin contributions cleanup + P0 inbox workspace picker

Plugin contributions:
- Remove sidebarItems from Activity, Journal (only workspace tabs)
- Remove workspaceItems from Browser Inbox (only sidebar/global)
- Remove workspaceItems from Search (only Ctrl+K command)
- Remove sidebarItems from Platform Test (developer tool, not in sidebar)

Browser Inbox:
- Add workspace picker for captures without workspaceRootPath
- Add assignToWorkspace() with proper storage key migration
- Add loadWorkspacesList() via plugin API
- Add picker CSS styles
- Selections now get same workspace assignment + conversion flow

Fixes P0 issues: captures and selections without workspace assignment
now have clear path to conversion (assign → convert)

plugins/search/plugin.json

@@ -27,14 +27,6 @@
     "entry": "frontend/src/index.js"
   },
   "contributes": {
-    "workspaceItems": [
-      {
-        "id": "verstak.search.workspace",
-        "title": "Search",
-        "icon": "search",
-        "component": "SearchView"
-      }
-    ],
     "commands": [
       {
         "id": "verstak.search.searchVaultText",