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/components/chat/work-status/WorkStatusMcpSection.tsx

@@ -13,6 +13,8 @@ type Props = {
   directory: string | null;
 };
 
+const MCP_STATUS_MAX_AGE_MS = 60_000;
+
 /**
  * MCP servers with their connection switches, reusing the dropdown's own
  * connect/disconnect actions.
@@ -23,17 +25,19 @@ export const WorkStatusMcpSection: React.FC<Props> = ({ directory }) => {
   const mcpStatus = useMcpStore(
     React.useCallback((state) => state.getStatusForDirectory(directory), [directory]),
   );
-  const refreshMcp = useMcpStore((state) => state.refresh);
+  const ensureMcpFresh = useMcpStore((state) => state.ensureFresh);
   const connect = useMcpStore((state) => state.connect);
   const disconnect = useMcpStore((state) => state.disconnect);
   const [busyServer, setBusyServer] = React.useState<string | null>(null);
 
   // The panel must not depend on the header dropdown having been mounted or
   // opened to know its MCP servers. Silent and background-gated, so it cannot
-  // compete with chat bootstrap traffic for sockets.
+  // compete with chat bootstrap traffic for sockets. The section remounts on
+  // every session switch, so it only asks for a status that is missing or
+  // older than a minute; connect/disconnect/auth refresh on their own.
   React.useEffect(() => {
-    void runBackgroundNetworkTask(() => refreshMcp({ directory, silent: true }));
-  }, [directory, refreshMcp]);
+    void runBackgroundNetworkTask(() => ensureMcpFresh({ directory, silent: true, maxAgeMs: MCP_STATUS_MAX_AGE_MS }));
+  }, [directory, ensureMcpFresh]);
 
   const mcpServers = React.useMemo(
     () => Object.entries(mcpStatus ?? {}).sort(([left], [right]) => left.localeCompare(right)),