refactor(app): reshape session store reads around narrow hook surface

Eliminate sidebar re-render cascades by fixing both sides of the
subscription contract:

- Session-store writes preserve identity: setWorkspaces, mergeWorkspaces,
  removeWorkspace, and patchWorkspaceScripts return the previous Map/entry
  reference when content is unchanged.
- HostRuntimeController.updateSnapshot only notifies when a patched field
  actually changed; idle probe ticks no longer replace the snapshot.
- New canonical hook surface in stores/session-store-hooks.ts
  (useWorkspace, useWorkspaceFields, useWorkspaceStructure,
  useWorkspaceKeys, useResolveWorkspaceIdByCwd,
  useWorkspaceStatusesForBadges, useWorkspaceExecutionAuthority,
  useRecommendedProjectPaths, useHasWorkspaces). These are the only
  place to subscribe to workspaces.
- 15 greedy subscribers migrated to the hook surface. The sidebar is now
  driven by useWorkspaceStructure for identity/ordering; each row
  hydrates its own descriptor via useWorkspaceFields, so a workspace
  update re-renders only its row. Archive-redirect callbacks switched to
  event-time useSessionStore.getState().
- Deleted dead code: use-session-directory.ts, workspace-fetch-debug.ts,
  and the unused SidebarProjectEntry aggregates (activeCount,
  totalWorkspaces, aggregated statusBucket).

No user-visible behavior change. Targeted tests green (79 tests across
host-runtime, session-store, session-store-hooks, sidebar list, sidebar
row model, sidebar shortcuts, workspace source of truth).

packages/app/src/screens/open-project-screen.tsx

@@ -8,6 +8,7 @@ import { MenuHeader } from "@/components/headers/menu-header";
 import { useOpenProjectPicker } from "@/hooks/use-open-project-picker";
 import { usePanelStore } from "@/stores/panel-store";
 import { useSessionStore } from "@/stores/session-store";
+import { useHasWorkspaces } from "@/stores/session-store-hooks";
 import {
   useIsCompactFormFactor,
   HEADER_INNER_HEIGHT,
@@ -22,7 +23,7 @@ export function OpenProjectScreen({ serverId }: { serverId: string }) {
   const openAgentList = usePanelStore((s) => s.openAgentList);
   const openProjectPicker = useOpenProjectPicker(serverId);
   const hasHydrated = useSessionStore((s) => s.sessions[serverId]?.hasHydratedWorkspaces ?? false);
-  const hasProjects = useSessionStore((s) => (s.sessions[serverId]?.workspaces?.size ?? 0) > 0);
+  const hasProjects = useHasWorkspaces(serverId);
   const isLocalDaemon = useIsLocalDaemon(serverId);
   const [isPairDeviceOpen, setIsPairDeviceOpen] = useState(false);