Leave the resolved decisions showing

The sidecar kept the record behind a closed disclosure, on the grounds
that it was worth keeping and not worth scrolling past. But it is what
the room settled, and a decision nobody can see is one that gets made a
second time — the pane's own argument for putting questions and accepted
comments in one list is that both are decisions, and it was then hiding
half of them by default.

The disclosure stays. Resolved threads are the record and are kept
forever, so a plan worked on for any length of time will out-scroll the
pane, and somebody who wants only what is still blocking them should be
able to say so.

Which way it is left is remembered, beside the pane widths. Collapsing it
and finding it open again on the next load reads as a toggle that does
not work. Anything other than the one stored string means open, so an
absent key and a corrupt one both give the default rather than a pane
that starts closed for a reason nobody can find. The hook is duplicated
rather than shared with `usePaneWidth`, which lives in `apps/web` and
cannot be reached from a package.

`aria-expanded` goes on the button while it is being touched, since the
glyph was the only thing saying which way it was. Nothing here is tested:
the pane needs a DOM to render into and the test runtime has none.