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/hooks/use-favicon-status.ts

@@ -1,13 +1,9 @@
 import { useEffect, useRef, useState } from "react";
-import { useShallow } from "zustand/shallow";
 import { getIsElectronRuntimeMac } from "@/constants/layout";
 import { useAggregatedAgents } from "./use-aggregated-agents";
 import { getDesktopHost } from "@/desktop/host";
-import { useSessionStore } from "@/stores/session-store";
-import {
-  deriveMacDockBadgeCountFromWorkspaceStatuses,
-  type DesktopBadgeWorkspaceStatus,
-} from "@/utils/desktop-badge-state";
+import { useWorkspaceStatusesForBadges } from "@/stores/session-store-hooks";
+import { deriveMacDockBadgeCountFromWorkspaceStatuses } from "@/utils/desktop-badge-state";
 import { isNative } from "@/constants/platform";
 
 type FaviconStatus = "none" | "running" | "attention";
@@ -99,17 +95,7 @@ async function updateMacDockBadge(count?: number) {
 
 export function useFaviconStatus() {
   const { agents } = useAggregatedAgents();
-  const workspaceStatuses = useSessionStore(
-    useShallow((state) => {
-      const statuses: DesktopBadgeWorkspaceStatus[] = [];
-      for (const session of Object.values(state.sessions)) {
-        for (const workspace of session.workspaces.values()) {
-          statuses.push(workspace.status);
-        }
-      }
-      return statuses;
-    }),
-  );
+  const workspaceStatuses = useWorkspaceStatusesForBadges();
   const [colorScheme, setColorScheme] = useState<ColorScheme>(getSystemColorScheme);
   const lastDockBadgeCountRef = useRef<number | undefined>(undefined);