Show a '+ New chat' action for empty chat sections in the Agents window

When a chat section in the Agents window has no chats, its current empty state is a non-interactive row labeled "No chats".

Instead, make the empty-state row actionable and label it "+ New chat" so users can start a chat directly from the section.

There is already a "New" button immediately above the section, so the design should avoid redundant controls. One option is to collapse chat sections by default when they are empty; expanding an empty section could then expose the "+ New chat" action.

_Created from a [Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1788066347873909?thread_ts=1788066347.873909&cid=C09MVCZKLP4)._