[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.

Start by locating the existing Desktop background-agent list and agent detail/header views, then trace the persisted runtime model metadata from sessions and usage events. Check the effective-model states introduced by #3824. Done means list rows and detail headers show the specified model states, including Auto routing, Unknown, and Mixed cases, without relying on color alone.

Written by the indexing model from the issue text.