When reopening an ACP agent thread (e.g. after restarting Zed) that has
no saved scroll position, the conversation now starts scrolled to the
bottom so the user sees the latest messages instead of starting at the
top.

Uses `set_follow_tail(true)` rather than a one-shot `scroll_to_end()` so
the scroll position is re-anchored on every layout pass — ensuring it
lands at the true bottom even when list items haven't been measured yet.
The follow-tail mode automatically disengages as soon as the user
scrolls up.

Release Notes:

- Fixed agent threads starting scrolled to the top when reopened after
restarting Zed.