[Desktop] Show effective model in background-agent list and detail views

> *This was generated by AI during triage.*

## Problem

Desktop Background agents list and detail views do not show which model performed the delegated work.

Task names sometimes mention a model, but they are user-authored and are not reliable metadata. Runtime model metadata is already persisted per session and usage event.

## Motivation

Model visibility helps users audit delegated work, compare agent runs, and understand Auto routing or differences in output.

## Proposed solution

Enrich the existing Desktop surfaces:

- **Background agents list:** show a short effective-model badge.
- **Agent detail/header:** show the model display name and stable model ID.
- Support truncation and tooltips; do not rely on color alone.

## Display semantics

- **Running:** latest effective model.
- **Completed:** effective model used.
- **Auto:** concrete routed model, with configured `Auto` optionally shown secondarily.
- **Unavailable:** `Unknown`; never infer from the parent session.
- **Multiple models:** `Mixed` in the list, with models shown per turn or as a list in details.

Terminology should align with the effective-model states introduced by #3824.

## Example workflow

1. Delegate several background agents using pinned and Auto-selected models.
2. Open **Background agents** to compare the effective models.
3. Open an agent to see its full model identity.

## Acceptance criteria

- List rows show the effective model when available.
- Detail headers show the display name and stable model ID.
- Auto-routed agents show the concrete effective model.
- Missing metadata displays `Unknown`.
- Multi-model agents display `Mixed`, with details available in the agent view.
- A live `Resolving...` transition is optional.

## Out of scope

- A new CLI/TUI worker panel.
- Reasoning-effort or context-tier display.
- Changing model selection or routing.
- Making the subagent aware of its model.

## Related issues

- https://github.com/github/copilot-cli/issues/1783 - requests a new CLI/TUI worker panel; this request concerns existing Desktop views.
- https://github.com/github/copilot-cli/issues/3824 - effective-model display in the CLI; useful parity context.

## Environment

- GitHub Copilot desktop app v1.0.25
- App-spawned CLI v1.0.71
- Windows 11

Cropped screenshots can be attached showing the missing model field in the Background agents list and agent header.

首先定位现有的 Desktop 后台代理列表以及代理详情/标头视图，然后从会话和使用事件中追踪持久化的运行时模型元数据。检查 #3824 引入的有效模型状态。完成标准是列表行和详情标头显示指定的模型状态，包括 Auto routing、Unknown 和 Mixed 情况，且不能仅依赖颜色。

由索引模型根据 Issue 内容生成。