fix(trajectory): reveal summary scrollbar thumbs on hover or focus

Summary scroll regions keep their scrollbar thumbs transparent until the
region is hovered or contains keyboard focus, without changing the
reserved scroll geometry. Updates the package README pair and the
inspection-ledger Agent Note together.

packages/client/ui-trajectory/tests/table.spec.tsx

@@ -109,6 +109,17 @@ describe('TrajectoryTable', () => {
     expect(screen.getByText('15 tok')).toBeTruthy()
   })
 
+  it('marks Summary scroll regions for interaction-only scrollbar thumbs', () => {
+    render(<TrajectoryTable turns={TURNS} {...FOLD_PROPS} />)
+    fireEvent.click(screen.getByRole('row', { name: /ASSISTANT/ }))
+
+    const panel = screen.getByRole('tabpanel')
+    expect(panel.querySelectorAll('[data-summary-scroll-region]').length).toBeGreaterThan(1)
+
+    fireEvent.click(screen.getByRole('tab', { name: 'Preview' }))
+    expect(panel.querySelector('[data-summary-scroll-region]')).toBeNull()
+  })
+
   it('keeps long thinking collapsed until the user asks to render it', () => {
     const thinking = 'private chain '.repeat(1_000)
     const turns: readonly TrajectoryTurnModel[] = [{