[FEATURE] Mod API: ui.render component for agent selector rows

Problem Statement

The agent selector under the prompt (● main, then one row per subagent or teammate) has no ui.render component, so a mod cannot restyle or extend a row. The only way today is to patch the minified bundle to wrap the row in a custom render site, and that patch breaks on most updates as anchors and compiled cache slots move.

Proposed Solution

A ui.render component per selector row, e.g. { component: 'AgentSelectorRow' }, with next(e) drawing the default row.

Useful props:

• agentId: the id $.agent.list() and agent.spawn use (none for main). The important one: it joins the row to its agent without guessing from the label.
• the row's label (agent type or teammate name) and title (summary or description)
• the agent's identity color, when it has one
• isSelected / isViewed, and the columns the row may take

Alternative Solutions

Patching the bundle, as above. A mod can also draw its own agent list in AbovePrompt or a Pane, but that duplicates the native selector instead of extending it.

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

1. A mod draws each row as a Powerline capsule in the agent's color, neutral for agents without one.
2. It appends · sonnet 5.5 to rows whose model differs from main's, read from $.agent.list() by agentId.
3. No patch, and both survive updates.

Additional Context

Same kind of ask as #101235 (welcome logo). Pairs with #102057 (model in AgentInfo). Version 2.1.299, terminal surface.