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