carry of #27611: transcript row tidy
Carry of #27611: agent transcript row tidy.
Original purpose: fix transcript spacing and weight, and make the sub-session row itself open its chat while its chevron toggles details. This carry keeps the original commits and merges current feat-cmux-next.
Original PR: https://github.com/manaflow-ai/cmux/pull/27611
No capture: the original PR's Linux gallery capture is advisory and cannot be reused as a private carry artifact; this carry changes no visual behavior beyond the already captured original diff.
Performance
No perf numbers: this carry changes CSS and click targets only; it adds no timer, observer, animation loop, or per-token work.
Takes back #24100: removes the detail-size and tracking declarations only with the obsolete .cv-agent-card__open selector, because Leo explicitly asked in #27611 to remove the "Open chat" label and make the row itself open its chat. Keeps bundled Inter, the shared type scale and calculated tracking, including the retained AgentCard model/status and doing text declarations; the new clickable header inherits the transcript typography.
Agent session: agent session