Give CollapsibleText a children-based variant and migrate AgentCard onto it

[atomantic]

client/src/components/cos/tabs/AgentCard.jsx's TaskDescription (~line 80) is a second "long task text with a Show more toggle" implementation, one directory over from the shared client/src/components/ui/CollapsibleText.jsx. It uses a text.length > 200 heuristic + max-h-[3.5rem] + gradient fade instead of measured line-clamp-2, and its toggle carries no aria-expanded/aria-controls.

Not consolidated when CollapsibleText was introduced because AgentCard renders markdown blocks via MarkdownOutput, which line-clamp can't clamp.

Decision: add a children-based max-height variant to CollapsibleText (an alternative clamp strategy for content that can't use CSS line-clamp, e.g. rendered markdown), then migrate AgentCard's TaskDescription onto it and delete the bespoke implementation. This unifies the collapse affordance and its accessibility (aria-expanded/aria-controls) everywhere, at the cost of visibly changing AgentCard's current fade treatment — call that out in the PR for review.

Deferred from the PR that clamped CoS task text.

Migrated from PLAN.md by /do:replan --issues.

[atomantic]

#4265 was merged while its review round was still in flight, so two of the three review findings did not land with it. They ship in follow-up PR #4272:

• hasChildren counted [] / '' as children, so a caller mapping an empty list got an empty capped box with its text fallback silently dropped.
• The collapsed container is a scroll container with no visible scrollbar, so tabbing to a link below the cap scrolled the preview out of reach with no way back, while aria-expanded="false" claimed content that was fully in the tab order was hidden. Focus inside now expands, gated on real overflow.

The third finding (effect deps missing the mode flag) landed on main in 6605399; #4272 adds the regression test it shipped without. Leaving this issue closed — #4272 is the remaining work.