Agents window: hierarchy connector lines and icon sizing are inconsistent for pinned sessions with multiple chats

In the Agents window, when a pinned session has multiple chats, the hierarchy is displayed messily:

- The line connecting the session to its chats is broken/disconnected instead of forming a continuous connector.
- The blue vertical line on the far left looks out of place / disconnected from the rest of the layout.
- The icon sizes are inconsistent between the pinned icon, the chevron, and the worktree name.

Overall this makes the pinned session hierarchy look messy and unpolished.

_Created from a [Slack conversation](https://vscodeteam.slack.com/archives/C09E1AKMB1P/p1787852641244029?thread_ts=1787852641.244029\&cid=C09E1AKMB1P)._