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/stores/session-store.ts

@@ -1,3 +1,4 @@
+import equal from "fast-deep-equal";
 import { create } from "zustand";
 import { subscribeWithSelector } from "zustand/middleware";
 import type { DaemonClient } from "@server/client/daemon-client";
@@ -159,6 +160,41 @@ export function mergeWorkspaceSnapshotWithExisting(input: {
   };
 }
 
+function preserveWorkspaceDescriptorIdentity(
+  incoming: WorkspaceDescriptor,
+  existing?: WorkspaceDescriptor | null,
+): WorkspaceDescriptor {
+  if (existing && equal(existing, incoming)) {
+    return existing;
+  }
+  return incoming;
+}
+
+function preserveWorkspaceMapIdentity(
+  existing: Map<string, WorkspaceDescriptor>,
+  incoming: Map<string, WorkspaceDescriptor>,
+): Map<string, WorkspaceDescriptor> {
+  if (existing === incoming) {
+    return existing;
+  }
+
+  const next = new Map<string, WorkspaceDescriptor>();
+  let changed = existing.size !== incoming.size;
+  const existingEntries = existing.entries();
+
+  for (const [key, workspace] of incoming) {
+    const existingWorkspace = existing.get(key);
+    const nextWorkspace = preserveWorkspaceDescriptorIdentity(workspace, existingWorkspace);
+    next.set(key, nextWorkspace);
+    const existingEntry = existingEntries.next().value;
+    if (!existingEntry || existingEntry[0] !== key || existingEntry[1] !== nextWorkspace) {
+      changed = true;
+    }
+  }
+
+  return changed ? next : existing;
+}
+
 export type ExplorerEntryKind = "file" | "directory";
 export type ExplorerFileKind = "text" | "image" | "binary";
 export type ExplorerEncoding = "utf-8" | "base64" | "none";
@@ -938,14 +974,18 @@ export const useSessionStore = create<SessionStore>()(
           }
           const nextWorkspaces =
             typeof workspaces === "function" ? workspaces(session.workspaces) : workspaces;
-          if (session.workspaces === nextWorkspaces) {
+          const preservedWorkspaces = preserveWorkspaceMapIdentity(
+            session.workspaces,
+            nextWorkspaces,
+          );
+          if (session.workspaces === preservedWorkspaces) {
             return prev;
           }
           return {
             ...prev,
             sessions: {
               ...prev.sessions,
-              [serverId]: { ...session, workspaces: nextWorkspaces },
+              [serverId]: { ...session, workspaces: preservedWorkspaces },
             },
           };
         });
@@ -962,13 +1002,15 @@ export const useSessionStore = create<SessionStore>()(
           let changed = false;
           for (const workspace of nextEntries) {
             const existing = next.get(workspace.id);
-            if (existing === workspace) {
+            const mergedWorkspace = mergeWorkspaceSnapshotWithExisting({
+              incoming: workspace,
+              existing,
+            });
+            const nextWorkspace = preserveWorkspaceDescriptorIdentity(mergedWorkspace, existing);
+            if (existing === nextWorkspace) {
               continue;
             }
-            next.set(
-              workspace.id,
-              mergeWorkspaceSnapshotWithExisting({ incoming: workspace, existing }),
-            );
+            next.set(workspace.id, nextWorkspace);
             changed = true;
           }
           if (!changed) {