fix(agent): capture full tmux scrollback for /usage scrape so the windows block is read (todo 2515)

The /usage overlay is taller than the pane, so the Current session / Current
week windows scroll into scrollback while visible-only capture-pane grabbed
only the lower What's contributing section. Switch the usage poll loop to
'capture-pane -p -S -' (full scrollback) and bump the ephemeral pane to -y 80.
Shared _capture_pane helper untouched -> claude_tui detach path unchanged.