Agent Host downloads Claude SDK 0.3.239 / Claude Code 2.1.239, but Fable 5.1 requires >= 2.1.251

### Environment

- Copilot Chat Extension Version: N/A
- `code --list-extensions --show-versions | Select-String "github.copilot"` returns no result.
- This is using VS Code's Agent Host / Claude Agent integration.
- VS Code Version: 1.136.2
- VS Code Commit: 88e44fa0e00b08f7758b4f6d05632e4fd5e4df6f
- OS: Windows x64
- Feature: Agent Host / Claude Agent
- Selected model: Claude Fable 5.1

### Problem

When starting a Claude Agent session with **Claude Fable 5.1**, VS Code's Agent Host downloads its own Claude Agent SDK into the Agent Host SDK cache, but the downloaded SDK contains **Claude Code 2.1.239**.

The session then immediately fails with the message that this model requires **Claude Code 2.1.251 or later**.

In other words, the same Agent Host path that downloads the Claude runtime downloads a version that is too old for a model already exposed in the model picker.

### Important diagnostic detail

The failing binary is not my system Claude Code installation and not the Anthropic VS Code extension binary.

The process launched by VS Code is:

```text
C:\Users\li\AppData\Roaming\Code\agent-host\sdk-cache\claude\0.3.239\win32-x64\node_modules\@anthropic-ai\claude-agent-sdk-win32-x64\claude.exe
````

That binary reports:

```text
2.1.239 (Claude Code)
```

For comparison, both other Claude Code installations on the machine are already newer:

```text
C:\Users\li\.local\bin\claude.exe
2.1.268 (Claude Code)

C:\Users\li\.vscode\extensions\anthropic.claude-code-2.1.268-win32-x64\resources\native-binary\claude.exe
2.1.268 (Claude Code)
```

So updating Claude Code itself or the Anthropic VS Code extension does not resolve the problem. The failing `2.1.239` binary is a separate copy managed/downloaded by VS Code Agent Host.

I confirmed the actual executable path with:

```powershell
Get-CimInstance Win32_Process |
Where-Object { $_.Name -eq "claude.exe" } |
Select-Object ProcessId, ParentProcessId, ExecutablePath, CommandLine |
Format-List
```

### Steps to reproduce

1. Open VS Code Copilot Agent / Claude Agent session on Windows.
2. Select **Claude Fable 5.1**.
3. Start an agent task.
4. VS Code Agent Host downloads/uses Claude Agent SDK `0.3.239` from `%APPDATA%\Code\agent-host\sdk-cache\claude\0.3.239\...`.
5. The bundled `claude.exe` reports version `2.1.239`.
6. The session fails because Fable 5.1 requires Claude Code `2.1.251+`.

### Expected behavior

If Fable 5.1 is available in the model picker, Agent Host should either:

* download/use a Claude Agent SDK whose bundled Claude Code satisfies the model's minimum version requirement; or
* hide/disable the model until the Agent Host runtime is compatible.

Ideally the dependency should be upgraded automatically instead of asking the user to update unrelated system/extension copies of Claude Code.

### Actual behavior

VS Code Agent Host downloads an incompatible runtime itself and then refuses to start the selected model because that runtime is too old.

This makes the error particularly confusing because manually updating every user-visible Claude Code installation has no effect on the Agent Host-managed SDK cache.