StatusbarHost laid every item at intrinsic width, so once the focused-pane
context line grew long (a model/mode/context/skills summary) the row's
content exceeded the bar width and overflowed instead of letting the slot
shrink. Add an opt-in flex factor to StatusItemContribution; the host wraps
flex>0 items in Flexible(loose) so they yield width when the bar is tight,
and the marquee then receives a bounded viewport and scrolls. Drops the
fixed maxWidth cap on the pane-context item.
T-160.
Co-Authored-By: Claude <noreply@anthropic.com>