T24f Agent: control and cost

HumanInputRequest asks a question with answers to pick and a line to
write one; CostBreakdown totals a session and splits it by part.
InterruptButton, TokenUsageChart, TraceViewer and EvalResultTable
point to chat::stop_button, a stacked BarChart, TimelineProfiler and
DataTable.

UsageBar shows amounts as its owner writes them. MemoryPanel and
HumanInputRequest share agent::line::send_line; the chapter's bool
callbacks take forms::OnFlag.

DataTable scrolls columns wider than its box sideways, pinned or not,
and leaves a plain wheel to the page; sortable headers are Tab stops.
TimelineProfiler fits its ruler to its lane, its spans are Tab stops,
and a picked span takes accent.

AGENTS.md

@@ -137,6 +137,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A flex line whose free space goes to an auto margin loses its gaps in taffy 0.9. A row that parts left and right groups its sides and uses `justify_between`; `ml_auto` is safe only beside a `flex_1` item that leaves no free space.
 - A header that holds a name beside counts and actions wraps: `flex_wrap`, the name `flex_1` with `min_w(label_width)`, the rest one `flex_none` group that drops below.
 - Every header, row and strip works at 280px: a name keeps its minimum width, the rest wraps below or scrolls, and nothing draws past its box.
+- gpui takes a scroll box's reach from its direct children's bounds, and a block child stretches to the box. Content that scrolls sideways sets its least width on that child (`tables::body::least`). An x-only scroll box also turns a plain wheel sideways unless `restrict_scroll_to_axis` is set through `style()`; a table's sets it, so the page keeps the wheel.
 - The focus color belongs to the focus ring. A chosen or shown item takes accent, as `forms::glyphs` and `lists::ListItem` do.
 - A box that shows a host's picture takes the picture's shape from the host (`ratio`), so nothing is cropped and a point given as a share lands on the picture.