feat(FN-4027): memoize startup slim list reads to avoid redundant fresh fet

Added memoization for startup slim task list reads (`store.ts`), with dashboard server routing startup reads through the memo window and tests covering the store watcher consumers in scheduler and worktree-pool, plus performance documentation.

Fusion-Task-Id: FN-4027

packages/engine/src/__tests__/scheduler.test.ts

@@ -1698,6 +1698,23 @@ describe("Scheduler", () => {
   });
 
   describe("pr monitoring", () => {
+    it("hydrates PR monitoring with startup memoized slim reads", async () => {
+      const prMonitor = {
+        startMonitoring: vi.fn(),
+        stopMonitoring: vi.fn(),
+        updatePrInfo: vi.fn(),
+        getTrackedPrs: vi.fn().mockReturnValue(new Map()),
+        stopAll: vi.fn(),
+      } as unknown as PrMonitor;
+
+      const store = createMockStore();
+      const scheduler = new Scheduler(store, {});
+      scheduler.configurePrMonitoring({ prMonitor });
+      await flushAsyncWork();
+
+      expect(store.listTasks).toHaveBeenCalledWith({ slim: true, includeArchived: false, startupMemo: true });
+    });
+
     it("stops monitoring when task moves out of in-review based on from column", () => {
       const prMonitor = {
         startMonitoring: vi.fn(),

packages/engine/src/__tests__/worktree-pool.test.ts

@@ -576,6 +576,7 @@ describe("scanIdleWorktrees", () => {
 
     const idle = await scanIdleWorktrees("/root", store);
 
+    expect(store.listTasks).toHaveBeenCalledWith({ slim: true, includeArchived: false, startupMemo: true });
     expect(idle).toContain("/root/.worktrees/calm-river");
     expect(idle).toContain("/root/.worktrees/bold-eagle");
     expect(idle).not.toContain("/root/.worktrees/swift-falcon");

packages/engine/src/scheduler.ts

@@ -516,7 +516,7 @@ export class Scheduler {
       return;
     }
 
-    void this.store.listTasks({ slim: true, includeArchived: false })
+    void this.store.listTasks({ slim: true, includeArchived: false, startupMemo: true })
       .then((tasks) => {
         const repo = getCurrentRepo(this.store.getRootDir());
         if (!repo) return;

packages/engine/src/worktree-pool.ts

@@ -304,7 +304,7 @@ export async function scanIdleWorktrees(rootDir: string, store: TaskStore): Prom
   const registeredDirs = dirs.filter((dir) => registeredWorktrees.has(resolve(dir)));
 
   // Find worktree paths assigned to non-done tasks (active worktrees)
-  const tasks = await store.listTasks({ slim: true, includeArchived: false });
+  const tasks = await store.listTasks({ slim: true, includeArchived: false, startupMemo: true });
   const activeWorktrees = new Set<string>();
   for (const task of tasks) {
     if (task.worktree && task.column !== "done" && registeredWorktrees.has(resolve(task.worktree))) {