Reasoning rendered as an always-expanded plain muted blob. Now it's a proper
collapsible part (opencode ReasoningPart): auto-EXPANDED while the turn streams
(watch it think), then collapses to a one-line `▶ Thought: <title>` when settled;
click toggles. Title is the model's leading `**bold**` line (reasoningSummary).
Body renders as DIM markdown in a left-`│`-border block (Markdown gained an
optional `fg` so reasoning is muted vs the answer). +1 render test (80 pass).
alt-glitch
6a24249e7c