Round-13 rebase onto main (2 commits). Main reverted the auto-settle
opt-in (#8596 undoing #8321), so the branch drops the ported
autoSettleMode machinery and returns to sidebarAutoSettleOnMerge with
settling-by-default, keeping only the v2 shell-shape delta in
threadSettled. The unpin confirmation (#7313) merges into the v2
thread-actions hook, and the mobile list tests re-sync to main's
reverted semantics in v2 shapes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>