[FEATURE] Mods on desktop: let a plugin add a card to the session panel

What

Desktop's right-hand session panel shows built-in cards (Environment, Context, Connectors, Changes). Let a mod add its own card there, drawn with the same elements as a Pane. For example, a ui.render component such as SessionPanelCard, opened with $.ui.card({ id, title }). The app would draw the card's title row and make it collapsible like the built-in cards; the mod would draw the body. Mod cards would sit after the built-in ones, and the person could collapse or hide them. In 2.1.300 no RenderComponent targets this panel.

Why

We run a mod that shows the current git worktree's Docker containers: running or stopped, health, an Open link, and Start/Stop/Restart buttons. That status belongs next to Environment, which already shows the worktree and branch. Today a mod has two options, and neither fits:

• The AbovePrompt band takes room above the prompt in every session.
• A $.ui.open Pane opens on desktop as a separate full-height side pane (placement: 'dock', about 55 columns). That's a lot of space for a few status rows.

A compact card would show the summary at a glance and could still open a pane for details.

Environment

Claude Code 2.1.300, Claude desktop app (Code tab), macOS.

Related: #101744 (split and rearrange mod panes on desktop).