Use asChild on CollapsibleTrigger to delegate to a plain button with
explicit border-0, bg-transparent, p-0 resets. Also align the left
margin with the time group headers (ml-2.5 instead of ml-0.5).

Replaces the raw <button> with manual style resets with the project's
Button component using variant=subtle, which already provides
border-none, bg-transparent, and the correct hover color.

Renders an <hr> with border-border-default between the time-grouped
active chats and the archived section. The divider only appears when
both active and archived agents are visible.

Replace the raw <hr> with the project's Separator component
(Radix-based, renders with bg-border) to match the rest of the site.

Replace the Button-based trigger with a plain div using the same
classes as the time group headers (Today, Yesterday, etc.):
mb-1 ml-2.5 flex items-center justify-between text-xs font-medium
text-content-secondary. Only additions are cursor-pointer and a
small chevron on the right side for the expand/collapse affordance.

Removes the ArchiveIcon and separate count badge from the trigger
in favor of inline 'Archived (N)' text that matches the simplicity
of the other section labels.

Also updates stories to match the new label format.

Wrap the active and archived sections in a div with divide-y
divide-border. The active group gets pb-2 and the archived
Collapsible gets pt-2 for symmetric spacing around the divider.

Removes the Separator component import and the manual conditional
that checked activeRootIDs.length.

Biome's formatter inserted literal space text nodes between JSX
elements. They're meaningless inside Collapsible/ChatTreeContext
and just add noise.

The asChild prop on CollapsibleTrigger uses React.Children.only,
which requires exactly one child. Previous formatting had multiple
elements concatenated on single lines, causing whitespace text nodes
to be parsed as extra children. Fixed by placing each element on its
own line.