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/activity/plugin.json

@@ -30,15 +30,7 @@
         "component": "ActivityView"
       }
     ],
-    "sidebarItems": [
-      {
-        "id": "verstak.activity.sidebar",
-        "title": "Activity",
-        "icon": "activity",
-        "view": "verstak.activity.view",
-        "position": 20
-      }
-    ],
+
     "workspaceItems": [
       {
         "id": "verstak.activity.workspace",