Any way for an extension to render a live panel in the terminal?

### Describe the feature or problem you'd like to solve

An extension has no way to render a live, refreshing panel in the terminal (e.g. a sub-agent progress dashboard) — `session.log` only appends scrolling text.

### Proposed solution

I'm building a CLI extension that runs a bunch of sub-agents in parallel and I want to show progress while they run. Something like:

```
my-run · explore · 3/5 done · 02:14
✓ auth-coverage done 45s 12.3K tok
✓ input-validation done 38s 10.1K tok
⠿ rate-limiting streaming 41s 3.2KB 8.7K tok
⠿ error-handling tool 41s (web_fetch)
· cors-policy pending 0s
```

ideally refreshing in place.

As far as I can tell there's no way to do this from an extension right now. The only output options are `session.log(..., { ephemeral: true })`, which is just scrolling text with no fixed region, and the modal `session.ui.*` dialogs. Since stdout is the JSON-RPC channel I can't draw to the terminal myself either.

I have all the data on the extension side (state, elapsed, tokens, current tool per agent) — I just have no surface to render it on.

Would you consider an API for extensions to render a panel like this in the TUI?
Even a read-only snapshot (no keyboard, no live push) would be enough to start — the extension hands the host some structured rows and the host draws them. Live updates could come later.

`@github/copilot` ~1.0.65, extension via `@github/copilot-sdk/extension`.

### Example prompts or workflows

_No response_

### Additional context

_No response_

まず、@github/copilot-sdk/extension を通じて公開されている extension API と、TUI で session.log および session.ui.* がどのように表示されるかを確認します。最小限の構造化行スナップショット API を最初に定義し、その後、extension が要求されたデータを提供できることと、host がそれを固定ターミナルパネルとしてレンダリングすることを検証します。ライブ更新とインタラクションは明示的に後の検討事項です。

索引モデルが issue の本文から書いたものです。