perf(chat): make session switching feel instant

Switching sessions ran as one synchronous commit: sidebar highlight, URL,
a full timeline remount with markdown re-parse, and around nine requests,
so nothing changed on screen for 150-250ms after the click.

- ChatContainer swaps the timeline on a deferred copy of the selection, so
  the active row, URL, and tab commit first and the timeline renders behind
  them; selection policy keeps reading the live store value.
- The message fetch starts before the selection is published.
- Sidebar rows stop re-rendering on a project switch: directory-scoped sync
  hooks read the runtime context and a subscribable current-directory source
  instead of the directory-bearing context; the grouping builder reads git
  branches through a ref and section caches key the branches they use;
  descendant ids are keyed by content. Rows per switch went from 73 to 8.
- Markdown skips the async re-render when the settled cached blocks are
  already painted, and mounts synchronously once its lazy module is loaded;
  the module is preloaded at boot.
- A timeline reveal gate holds a freshly opened session at opacity 0 while
  any provisional markdown paint catches up (250ms cap), then fades the whole
  timeline in once, so text, tools, and recap appear together.
- Switch fan-out trimmed: knowledge summary deduped, MCP status refreshed only
  when stale, non-repo directories cached by the git repo check, OpenChamber
  defaults cached briefly, agent memory reused for the same project, goal
  text cached, PWA manifest rebuilt after the switch settles.
- Header tabs snap into the active state and keep the title at the same
  height in both states.
- Prefetch on row press; composer focus moved off the commit.

`bun run profile:switch` records ack/content latency, longest task, and
requests per switch, cold and warm, and compares runs against a baseline.
Measured warm switch: ack 228ms to about 40-60ms, content 228ms to about
100-120ms.

packages/ui/src/hooks/useAgentMemorySync.ts

@@ -23,6 +23,8 @@ import { useProjectContextOwner } from '@/hooks/useProjectContextOwner';
  * because this runs above `SyncProvider` — that hook reads the sync context and
  * throws outside it, which took the whole app down with a blank window.
  */
+const AGENT_MEMORY_FRESH_MS = 60_000;
+
 export const useAgentMemorySync = (directory: string | null): void => {
   const enabled = useUIStore((state) => (
     state.agentMemoryFeatureAvailable && state.agentMemoryToolEnabled
@@ -31,11 +33,14 @@ export const useAgentMemorySync = (directory: string | null): void => {
   const owner = useProjectContextOwner(directory);
   const projectPath = owner?.path ?? null;
 
+  // The owner re-resolves on every directory switch; entries loaded moments
+  // ago for the same project are still current, and the change event below
+  // forces a re-read when the agent writes memory.
   React.useEffect(() => {
     if (!enabled) {
       return;
     }
-    void load(projectPath);
+    void load(projectPath, { maxAgeMs: AGENT_MEMORY_FRESH_MS });
   }, [enabled, load, projectPath]);
 
   // The agent writes memory mid-turn through its own tool, so the index for the

packages/ui/src/hooks/usePwaManifestSync.ts

@@ -14,6 +14,7 @@ type ManifestSyncWindow = Window & {
 };
 
 const MAX_RECENT_SHORTCUTS = 3;
+const MANIFEST_UPDATE_DELAY_MS = 2_000;
 
 const normalizeRecentTitle = (value: string | undefined, fallback: string): string => {
   if (typeof value !== 'string') {
@@ -86,7 +87,13 @@ export const usePwaManifestSync = () => {
       return;
     }
 
-    const win = window as ManifestSyncWindow;
-    win.__OPENCHAMBER_UPDATE_PWA_MANIFEST__?.();
+    // Rebuilding the manifest fetches it from the server. Shortcuts only
+    // matter to the installed-app menu, so the rebuild waits until the switch
+    // that changed them has settled instead of adding a request to it.
+    const timer = window.setTimeout(() => {
+      const win = window as ManifestSyncWindow;
+      win.__OPENCHAMBER_UPDATE_PWA_MANIFEST__?.();
+    }, MANIFEST_UPDATE_DELAY_MS);
+    return () => window.clearTimeout(timer);
   }, [hasRecentShortcuts, signature]);
 };

packages/ui/src/hooks/useSessionGoal.ts

@@ -22,6 +22,9 @@ export function useSessionGoal(sessionId: string, directory?: string): SessionGo
   };
 }
 
+const OBJECTIVE_CONTENT_CACHE_MAX = 64;
+const objectiveContentByFetchKey = new Map<string, Promise<string | null>>();
+
 // Effective objective text for display. Inline goals return the metadata
 // text directly; file-backed goals fetch the server-side file once per
 // goal edit (keyed by id + updatedAt). Display-only: a failed fetch yields
@@ -37,7 +40,18 @@ export function useGoalObjectiveContent(sessionId: string, goal: SessionGoalPayl
       return undefined;
     }
     let alive = true;
-    void fetchGoalObjectiveContent(sessionId).then((content) => {
+    // The key already names the goal edit, so a remount (every session switch
+    // remounts the strip) reuses the text instead of fetching the file again.
+    let request = objectiveContentByFetchKey.get(fetchKey);
+    if (!request) {
+      request = fetchGoalObjectiveContent(sessionId);
+      objectiveContentByFetchKey.set(fetchKey, request);
+      if (objectiveContentByFetchKey.size > OBJECTIVE_CONTENT_CACHE_MAX) {
+        const oldest = objectiveContentByFetchKey.keys().next().value;
+        if (oldest !== undefined) objectiveContentByFetchKey.delete(oldest);
+      }
+    }
+    void request.then((content) => {
       if (alive) setFetched(content);
     });
     return () => {