feat: workspace frontend core (PR-3)

- Workspace types, API client, hooks (useWorkspaces, useWorkspaceActions)
- WorkspaceCard, WorkspaceCreateForm, StartToolModal components
- WorkspacesPage with list, create, sync, delete, start-tool flow
- Sidebar navigation: new 'Workspaces' entry
- Router: /workspaces route
- TypeScript + eslint clean

apps/web/src/components/app-shell.tsx

@@ -24,6 +24,7 @@ const NAV_ITEMS: {
 }[] = [
 	{ to: "/", label: "Home", icon: "dashboard" },
 	{ to: "/sessions", label: "Sessions", icon: "terminal", badge: "sessions" },
+	{ to: "/workspaces", label: "Workspaces", icon: "folder" },
 	{ to: "/projects", label: "Projects", icon: "projects" },
 	{ to: "/tool-workshop", label: "Tool Workshop", icon: "settings" },
 	{ to: "/config-profiles", label: "Config Profiles", icon: "folder" },