fix(chat): open a session already at its end, and keep it there

A session opened from the sidebar could land above the bottom, or show a
frame sitting lower and then snap up. The viewport was pinned before the
content was final: the recap note renders once the session record arrives
and grew the footer under the pinned viewport, and on large sessions
subagent task cards grow when their child sessions load, moving everything
above the viewport.

- The recap note holds the timeline reveal until the session record is in
  memory, so it is part of the first finished picture.
- The scroll hook holds the reveal until the viewport is pinned; the reveal
  itself runs once the content height has held still for two frames, with
  one exact pin against the final height (bounded at 300ms).
- Sitting on the end of a session that is not producing output is an
  invariant: content growth re-pins from a MutationObserver in the same
  frame the list writes its layout, so no frame paints with the end out of
  view. Output growth keeps gliding through followEnd, which now glides only
  while the session is working.

packages/ui/src/components/chat/timelineRevealGate.ts

@@ -13,7 +13,7 @@ import React from 'react';
  * A hold that never releases must not hide the chat forever, so the owner
  * reveals after `TIMELINE_REVEAL_CAP_MS` regardless.
  */
-type TimelineRevealGate = {
+export type TimelineRevealGate = {
   /** Take a hold; returns the release. Returns null once the gate is closed. */
   hold: () => (() => void) | null;
   /** Stops accepting holds. Existing holds still count. */