style(agent): redesign AgentPane and MarkdownPane for polished readable UI
Tribunal-elected design (S-3-R4, 88% confidence). AgentPane: sans-serif root
font, tool call/result pairing via $derived.by, hook message collapsing,
context window meter, minimal cost bar, session summary styling, two-phase
scroll anchoring, tool-aware truncation, color-mix() softening, container
query responsive margins. MarkdownPane: container query wrapper. Shared
--bterminal-pane-padding-inline CSS variable in catppuccin.css.
4674a4779d