Long tool and subagent cards should have max height + scrollbar

[osimarr]

Body

Problem

Long expanded ACP cards, such as tool output or subagent activity, can grow to fill the Agent Panel. When this happens, the rest of the conversation becomes difficult to browse, especially as output continues to stream.

Requested behavior

• Let short card content use its natural height.
• Cap the card content area at a maximum height. When content exceeds it, show a usable vertical scrollbar inside the card so the full output remains accessible.
• Keep the card header and actions accessible, and allow the surrounding conversation to scroll normally.
• Apply this to ACP tool and subagent cards, including cards whose content grows while streaming.

A configurable maximum with a sensible default would help accommodate different panel sizes.

Expected result

A user can scroll to the end of a long ACP card and still browse messages before and after it in the Agent Panel.

[osimarr]

This is the problem: a card can take the whole screen and more. It's hard to navigate the window.